How to build a UHF antenna... - Page 130 - AVS Forum | Home Theater Discussions And Reviews
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post #3871 of 4912 Old 04-18-2010, 06:03 PM
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As an aside, you can't combine "deep-fringe" and omni-directional on the same antenna. To get high gain in one direction, you must have very low gain in many others. Can't fight physics.
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post #3872 of 4912 Old 04-18-2010, 06:10 PM
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Yeah, I kinda understand that, but fussing with a rotator is not something I'm interested in. I'm just looking for something that performs better than the copper cactus j-pole I built, or the ordinary twin lead FM dipole.
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post #3873 of 4912 Old 04-18-2010, 06:53 PM
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MLords antenna seemed interesting. (The one made with steel wire on black ABS plastic), as was the vertical folded dipole made out of copper tubing.

They both seem like giant versions of a twin-lead dipole.- Probably have more bandwidth, too.

What would be the best, proper dimensions for that 1/2" copper pipe dipole for 88-108 MHz FM band? (Total length, center to center top and bottom gap, and center to center end gap.) It seems the one in the article is for VHF. Also, am I correct in assuming that that type of antenna should be inverted horizontally, as opposed to vertically like the J-Pole. It seems like it should be this way, just like a 300 ohm twin-lead dipole goes straight up, then branches out into a "T." Would that be correct? Also, is this a "bi-directional" antenna or "omni-directional? Does it have more gain in front of it and behind it, than off to the sides of the long horizontal pipe? The reason I ask is bi-directional would be OK, as I have 4 stations I would be most interested in getting, and there are 2 in one direction, and two in directly the opposite direction.

Many thanks again, for all ther helpful links. I appreciate the great information.

Warm Regards,
Eric

I get the impression the "copper cactus" J-Pole would be all wrong for FM radio reception, and more appropriate for Ham.

I could use the material from my J-Pole to rebuild it into a vertical dipole.
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post #3874 of 4912 Old 04-19-2010, 12:18 AM
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Also somewhat interested in that Circular Polarized Omni antenna.

Regardless of what antenna I wind up using, I am wondering if there is any benefit to be had in gain by possibly using some of that 14 guage ladder wire like I made for my DBGH as the antenna wire, directly from my receiver to an antenna for an ultra-low-loss antenna lead.

I'm also brainstorming about the idea of constructing a twin lead dipole out of 14 or even 12 AWG wire stripped down, and mounted on a PVC pipe, like the one MLord built, and putting it up 20' in the air fo increased line of sight. Think that would work well?

Would this likely give me any better FM reception than an ordinary, "T" shaped, thin guage 300 ohm twin lead dipole indoors? Could I cut it to the same dimensions as I would a thin guage one, or will those dimensions need to be modified?

Im wondering what size PVC pipe to use, and how long it should be in order to achieve the proper length/spacing for roughly the best comprimise between 88-108 Mhz on the FM band.

Holl_ands, Please let me know if this idea seems viable, or if there is a better alternative like some of those others mentioned.
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post #3875 of 4912 Old 04-19-2010, 11:13 AM
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Quote:


I am wondering if there is any benefit to be had in gain by possibly using some of that 14 guage ladder wire like I made for my DBGH as the antenna wire, directly from my receiver to an antenna for an ultra-low-loss antenna lead.

Sure, especially if the FM receiver has 300 ohm inputs.

Quote:


Would this likely give me any better FM reception than an ordinary, "T" shaped, thin guage 300 ohm twin lead dipole indoors?

Anything outdoors will give you better reception than indoors, except in very weird circumstances.

Quote:


Im wondering what size PVC pipe to use, and how long it should be in order to achieve the proper length/spacing for roughly the best comprimise between 88-108 Mhz on the FM band.

The plastic pipe is just for support, so the diameter is determined by the dipole spacing. Download the program Yagi Calculator and choose the folded dipole option.
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post #3876 of 4912 Old 04-19-2010, 11:28 AM
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FM Tuner Sensitivity & Selectivity:
Marantz 2275 specs only cite a MONO sensitivity of 1.9 uV [-109.2 dBm, 8.7 dBf]
(nothing mentioned re STEREO) and an Alternate Channel Selectivity of 80 dB:
http://www.classic-audio.com/marantz/2275.html
Clearly a typo in the above table: "Adjacent" should be "Alternate", as in "similar" 2325:
http://www.classic-audio.com/marantz/2325.html
http://www.fmtunerinfo.com/receivers.html
Marantz 2325 specs claim 5.0 uV [-100.8 dBm, 19.2 dBf] MONO for a (more or less)
listenable 50 dB SNR (ultimate is 70 SNR).
Unfortunately, Marantz specs don't say anything about STEREO performance,
although some "similar" units cite 35 uV [-83.9 dBm, 36.1 dBf] for 50 dB STEREO SNR....

My 1971 Kenwood KT7001 design is "similar" with 4-gang tuner and crystal IF filters:
http://www.fmtunerinfo.com/kenwood.html
Kenwood specs only say 1.5 uV [-111.3 dBm, 8.7 dBf] IHF Sensitivity (MONO, 30 dB SNR)
(nothing re STEREO) and 90 dB Alternate Channel Selectivity, just slightly better than 2325.
Back in 1976, I brought my KT7001 to a Kenwood Tuner Clinic held at a local
Hi-Fi Convention....which was also demonstrating the "Earthquake" sound system.

Following are test results, per (non-Kenwood) techs (alas the 7001 died a few years ago):
MONO:
1.6 uV for 30 dB SNR (IHF Sensitivity) [-110.7 dBm, 9.3 dBf]
2 uV for 50 dB SNR [-108.8 dBm, 11.2 dBf]
6 uV for 60 dB SNR [-99.2 dBm, 20.8 dBf]
73 dB ultimate SNR, MONO (1000 uV) [-54.8 dBm, 65.2 dBf]
0.14% Total Harmonic Distortion (1000 uV) [-54.8 dBm, 65.2 dBf]
0.9% THD at 50 dB SNR (2 uV) [-108.8 dBm, 11.2 dBf]
STEREO:
4 uV for 30 dB SNR [-102.8 dBm, 17.2 dBf]
40 uV for 50 dB SNR [-82.8 dBm, 37.2 dBf]
140 uV for 60 dB SNR [-71.9 dBm, 48.1 dBf]
70 dB ultimate SNR, STEREO (1000 uV) [-54.8 dBm, 65.2 dBf]
38 dB STEREO separation at 1000 Hz
1.0% THD (1000 uV) [-54.8 dBm, 65.2 dBf]
1.0% THD at 50 dB SNR (40 uV) [-82.8 dBm, 37.2 dBf]

In the brackets, I converted uV to dBm (300-ohm) levels, presumably in
accordance with the OLD test procedure (cuz some didn't have 75-ohm).
I'm not sure whether FMFool uses 300-ohm reference or 75-ohm reference.
If FMFool is using 75-ohm, then add 6 dB to the above 300-ohm levels.

Modern FM Tuners use a NEW test procedure, with sensitivity in dBf (re 1 femtowatt),
which is the SAME whether measured across 75-ohms or 300-ohms.
Subtract 120 db from a dBf number to convert to dBm.

Lets compare a -85 dBm (STEREO, 50+ dB SNR) sensitivity level to your FMFool results.
All stations above the "grey" band "should" be receivable, presuming a 2 dBi Gain antenna.

Those stations in the grey band will be very difficult, esp. when they are within
two channels of a strong station. Marantz 2525 is much better prepared to
receive an station that is two channel positions away (next adjacent....or "alternate")
than the older tuners, which might only have 40-50 dB selectivity....vice 80 dB.

A directional antenna would help to suppress the stronger stations.

FM Third Order Intermods:
You have three fairly strong local FM stations. Although I don't recall
seeing any overload tests on FM Tuners, fol. stations "MIGHT" be affected:
99.5, 100.3, 100.5, 101.5, 102.1, 103.3, 104.1 & 104.9 MHz.
[I modified my DTV IMD Calculator spread sheet....]
Local stations on 99.5, 100.5 and 104.9 won't be affected cuz they're so strong.
W262AI on 100.3 and especially very weak WAEB-FM on 104.1 "MIGHT" be affected.

A directional antenna (with a rotator) could be used to reduce the signal
levels from the local stations to reduce the effects of intermod generation.

FM Man-Made Noise/Interference:
Ultimately, whether you receive a particular station or not will probably depend on
the levels and characteristics of Man-Made Noise/Interference. The FMFool
(as well as TVFool) calculations do NOT include a guestimate for the widely varying
levels of impulse....and broadband noise emanating from ignition systems,
fluorescent lights, dimmer switches, brush-type motors, ad nauseum.....so YMMV....

A directional antenna can significantly reduce noise pickup from directions other
then the desired signal direction.....again, YMMV.....

Comments re 300 vs 75-ohm Inputs:
In my KT7001, the 75-ohm coax input is DIRECTLY connected to the first
RF tuner stage whereas the 300-ohm input goes through a balanced
(floating input) to unbalanced Balun. Hence lower loss using 75-ohm input.

However in the earlier KT7000, there were two Baluns in parallel: one for 75-ohm
and the other for 300-ohm input, which had one of the "balanced" input leads
inexplicably connected to GROUND. Balun loss was probably about the SAME...

If you look inside any FM Tuner with a 300-ohm input, you are going to
find an internal Balun to convert from unbalanced to balanced, as well as
reducing the impedance for the RF input stage. The loss is probably 0.5 dB.

Hence, when using a 300-ohm impedance antenna, there is only ONE
(external) Balun to worry about if the Tuner has a direct connection on
the 75-ohm input (check your schematics). And for sure there will be
TWO Baluns in series if you use the 300-ohm input. FWIW, all of my
more modern AVRs only have 75-ohm inputs....which can be misleading
when spec numbers are STILL referenced to uV (across 300-ohm!!!).
Thankfully, they also state sensitivity in dBf, independent of impedance.

If you have an older FM Tuner that only has 300-ohm input, twin-lead
can eliminate the need for a (0.5 dB loss) Balun at the antenna, and
might have a slightly lower cable loss....but when it rains/snows the
loss of twin-lead increases, whereas coax does not:
http://www.saarsham.net/coax.html
http://www.avsforum.com/avs-vb/showt...php?p=16796125
http://www.avsforum.com/avs-vb/showt...php?p=15969694
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post #3877 of 4912 Old 04-19-2010, 01:08 PM
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Since you mentioned H H Scott Tuners, thought I would call attention
to a late-30's E H Scott Receiver that my H.S. and College Physics Lab
partner and later co-worker resurrected after many hours of work.
It was just the inner, shiny chrome plated guts from a E H Scott
Philharmonic All-Band Receiver Console...now THAT'S a Receiver:
http://www.radioblvd.com/ConsolePhoto.htm
http://www.antiqueradio.com/images/D...st-EHScott.jpg
http://www.antiqueradio.com/May04_Br..._Quaranta.html
http://classicradiogallery.com/wood_m-z.html
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post #3878 of 4912 Old 04-19-2010, 02:56 PM
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Quote:
Originally Posted by nepaeric View Post

MLords antenna seemed interesting. (The one made with steel wire on black ABS plastic), as was the vertical folded dipole made out of copper tubing.

They both seem like giant versions of a twin-lead dipole.- Probably have more bandwidth, too.

What would be the best, proper dimensions for that 1/2" copper pipe dipole for 88-108 MHz FM band? (Total length, center to center top and bottom gap, and center to center end gap.) It seems the one in the article is for VHF. Also, am I correct in assuming that that type of antenna should be inverted horizontally, as opposed to vertically like the J-Pole. It seems like it should be this way, just like a 300 ohm twin-lead dipole goes straight up, then branches out into a "T." Would that be correct? Also, is this a "bi-directional" antenna or "omni-directional? Does it have more gain in front of it and behind it, than off to the sides of the long horizontal pipe? The reason I ask is bi-directional would be OK, as I have 4 stations I would be most interested in getting, and there are 2 in one direction, and two in directly the opposite direction.

Many thanks again, for all their helpful links. I appreciate the great information.

Warm Regards,
Eric

I get the impression the "copper cactus" J-Pole would be all wrong for FM radio reception, and more appropriate for Ham.

I could use the material from my J-Pole to rebuild it into a vertical dipole.

I wouldn't say the J-Pole was all wrong for FM....it's roughly equal to a Vertical Dipole,
but comes with it's own bag of impedance matching problems....which are NOT described
in the plans when trying to adapt it for use with 75-ohm or 300-ohm systems....
[And I'm trying to avoid volunteering, since I don't have a SWR test apparatus.]

A classic twin-lead Folded Dipole for FM is usually shown with a vertically
orientated twin-lead connecting to a horizontally disposed "T" section:
http://www.wfu.edu/~matthews/misc/dipole.html
However, only the horizontal "T" section at the top is an active part of
the antenna....the lower portion feeding the antenna could be arranged
anyway that's convenient, as long as it's kept away from other objects,
including zig-zaging around obstacles or running horizontally across the
roof (or floor) to the antenna....whatever....

A Dipole or Folded Dipole will have a bi-directional pattern when Horizontal:
http://www.signalengineering.com/ultimate/dipolerad.jpg
which means nulls wherever the ends of the wires "point".

When it is rotated around so that it is Vertical, it has an omni-directional
pattern, with the nulls harmlessly directed straight up and down:
http://en.wikibooks.org/w/index.php?...20080803235806
http://en.wikipedia.org/wiki/Fileipolentstehung.gif
But it will now respond only to Vertically (or Circularly/Elliptical) Polarized transmissions,
which are typically found in FM Band (and many more coming on-line for Mobile DTV).
In Free Space there is no difference in Gain between Horiz. and Vert. Dipole.
However, when the Earth's Ground Plane is considered, the Vert. Dipole
might have a 1 dB advantage....maybe....might also work better thru trees.
[Notice I'm hedging here...depends on the kinds of trees & ground conductivity...]

I analyzed various wire/tubing sizes for Folded Dipoles here:
http://imageevent.com/holl_ands/loops/folded
The Hi-VHF charts (center ~200 MHz) can be rescaled for FM Band (center ~100 MHz)
by doubling the wire sizes. Hence Hi-VHF chart for 1/4-in O.D. becomes 1/2-in for FM.
You can also slide the Ch6/FM chart upwards into the FM Band to see that very thick
antenna elements are needed in the FM band for best SWR.....twin-lead is very poor
with a very limited bandwidth of "good" performance.

Fortunately, in the FM Band, SWR is of lesser importance than for DTV....but will
still cause nulls along the transmission line every half-wavelength, which we
quantify as the difference between Raw Gain and Net Gain:
http://en.wikipedia.org/wiki/Standing_wave_ratio

Note that if the wires forming the Folded Dipole are equal, the separation
between them only mildly affects performance (no, it is NOT zero difference).
As separation increases, the Folded Dipole length decreases to compensate.
I ran several combinations for Hi-VHF, which can be rescaled for FM Band
by simply multiplying ALL dimensions by 2 (including element diameter).
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post #3879 of 4912 Old 04-19-2010, 10:41 PM
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Thankyou Holl_ands and 300 ohm for the wealth of info.

I will begin by trying a few of these various FM antennas discussed and outlined in these links you provided. I will also likely begin with ultra-low-loss300 ohm ladder wire I will construct again, out of 14 guage solid wire, in light of the fact of having only 300 ohm inputs on the receiver. This will also eliminate any loss issues with matching for use of 75 ohm coax, necessitating the use of 2 baluns.- (one at the antenna, and one at the receiver's 300 ohm inputs.) I will likely try to run it up to the roof through some PVC conduit to eliminate any problems associated with contact with metal and/or moisture from precipitation which can degrade results. Then I will test various antennas outlined here, and experiment with them all until I acheive optimum results, and I will post them here.

Yes, Holl_ands, I agree. You cannot beat those early vintage receivers and tuners/amplifiers. High quality these days, has been replaced by high-tech, digital, bells and whistles, and low-quality.

One thing I have noticed about my Marantz's, compared with modern day equipment is that their tuner performance is superior, in Stereo or Mono Mode. (I compared one to a modern one using the same antenna and location.) Rejection of interference with adjacent channels is better, and I can fine tune my "Gyro-tuner," and use my signal strength meters, and other features to obtain a more perfected precision in tuning than with a modern stereo receiver.

Many thanks, again, Holl_ands and 300 ohm.

Warm Regards,
Eric
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post #3880 of 4912 Old 04-20-2010, 06:40 AM
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Just wanted to give a quick update on the horizontal stack. I finished building and installing the reflector at about 11 last night. The horizontal mounts for the grills gave me fits, mainly because there were some problems I should have foreseen that I didn't.

So, in the dark, I planted the antenna in the front garden at ground level (I've developed a pretty good feel for where magnetic 139 is) and was able to grab 11 stations, including 4 I'd never been able to get. I now get a fantastic signal on 9, 11, 28, and 35. However, 22 and 39 are still not coming in for some reason. I don't know if it has something to do with the scaling or construction on my antenna, but these two theoretically should have come in before I ever thought about getting 35, if my TVfool report is to be trusted.

I should note that I flattened my elements for this version of the antenna. With the flat reflector, I wasn't sure what effect I would get from forward swept elements. I'll post pictures later. The frame still needs some shoring up, but I was excited to try it out.
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post #3881 of 4912 Old 04-20-2010, 09:13 AM
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Being able to MANUALLY detune away from interference is an important, but "LOST" feature....

I noticed your Marantz 2275 FM Tuner didn't have switchable WIDE/NARROW selectivity.
Here are specs for some of the better, switched filter, FM Tuners:
http://www.accuphase.com/cat/t-105en.pdf [Fixed 0.1 MHz tuning steps--minimal detuning!!!]
http://www.anarc.org/wtfda/cartx11.pdf [Fixed 0.05 MHz tuning steps--better detuning.]
Note that both specs clearly understand difference in uV specs for 75 and 300-ohm inputs.

And here's how to mod certain FM Tuners for even MORE selectivity:
http://www.anarc.org/wtfda/fmfilter.pdf

FYI: More info re "best" FM Tuners (and tuner mods) for DXing:
http://home.iprimus.com.au/toddemsli...erforfmdx.html
http://www.anarc.org/wtfda/treasury_...l_articles.htm
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post #3882 of 4912 Old 04-20-2010, 10:25 AM
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Quote:


You cannot beat those early vintage receivers and tuners/amplifiers.

A lot of people still like the "warm" sound from vacuum tube amplifiers.

For cheap AM/FM dxing, quality car radios hooked up to high gain antennas and low noise power supplies are hard to beat.
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post #3883 of 4912 Old 04-20-2010, 11:18 AM
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Quote:
Originally Posted by jstarling82 View Post

Just wanted to give a quick update on the horizontal stack. I finished building and installing the reflector at about 11 last night. The horizontal mounts for the grills gave me fits, mainly because there were some problems I should have foreseen that I didn't....

Took it up to the roof mount during my lunch break and completely annihilated my signal. With the more narrow beam width, the chimney and roof peak are in the way, I suppose. No more peeking in between the two with this configuration.
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post #3884 of 4912 Old 04-21-2010, 10:55 AM
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I set this in a slightly different spot in the front yard, and tilted and turned it a little, and I was finally able to find the elusive RF 39. So, with this antenna, I know I will be capable of receiving 9, 11, 28, 35, and 39. 22 is all that's missing, but I'm confident that with a little elevation, I'll find it out there somewhere. It's right in the middle of everything else I'm getting, and I got it with the reflectored 4-bay, so why not?

Pics attached!
LL
LL
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post #3885 of 4912 Old 04-21-2010, 11:34 AM
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Nice use of the cooling racks for reflectors, isn't it funny (odd) how the reception changes for one spot to the other. Goes to show how important location is to reception. I can't explain why the 4 bay gets ch 22 but neither 8 bay you built will.
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post #3886 of 4912 Old 04-21-2010, 01:04 PM
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Thanks! They're not precisely level, and I still have to figure out a way to keep the outer edges on the same plane, but I think it's getting there.

Yeah, I moved the antenna about 2 feet closer to the transmitters (39 miles minus 2 feet), and turned it slightly further south to catch a break in the pines. I will say that the 4-bay (with reflector) got 22 from the back corner of the house, which is no longer a good spot, since the 4-bay peered between the roof and chimney...something neither 8-bay is capable of doing. However, I've yet to try this version on the face of the chimney or nearer the roof peak, so I'm expecting good things in the near future...hopefully very near, since I've been working on this for about two months.
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post #3887 of 4912 Old 04-21-2010, 01:18 PM
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So far, all I was able to accomplish was the assembly of a simple, easy to construct folded dipole. I made it out of 1/2" plastic trim molding material, so it is very "floppy." I strung 12 guage copper, (stipped down the insulation first) around a piece I cut down to 54-1/2".(as per calculator length for FM- 103 MHz) Then I ran it up the roof and put it about 20-25' in the air, directly to the Marantz receiver via standard 300 ohm twin lead. ( I did not make any 14 guage ladder wire yet.) Signal strength has improved slightly over the former J-pole and the ordinary 300 ohn twinlead dipole mounted indoors.

My next project, I would like to try building first, the folded dipole out of 1/2" copper tubing, but I am not sure the length that is optimum for:
1. The FM band (88-108MHz)
2. FM 103MHz, which is about an average of 2 stations I am trying to get most.

I am also not sure the exact dimensions, such as top and bottom, center to center, and feed point gap spacing. Although it appears that butting the two elbows gives you roughly the top and bottom spacing.

My next objective will likely be to then add this dipole to an array of directors and reflectors in a yagi format to see how this being more directional might improve any problems with interference. Sound like a plan? Holl_ands and 300 ohm, I'd reall appreciate your input on this.

Regards,
Eric
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post #3888 of 4912 Old 04-21-2010, 02:22 PM
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I will be mounting these 2 puppies soon and will post some results.

The 8 bay is a unique design and preliminary results show it to be as good as my CM4228 if not better on all channels. I know you technophiles will laugh and point out all the non conformance with your specs...but it seems to work really well.

THe VHF is a standard design that I built with used antenna parts. Here again the reception from a test I did appears to be quite good.

Both antennas used TREXX deck scraps for mounting the whiskers
LL
LL
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post #3889 of 4912 Old 04-21-2010, 02:23 PM
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FM Folded Dipole sizes using 1/2-in Copper Tubing are either 53.0 x 8.0 inch
or 55.0 x 4.0 inch, where first number is the end-to-end length and the second
number is the separation between the long tubes (center-to-center).
This results in SWR=2.0 at 88 & 108 MHz and minimum of 1.04 mid-band.

You can do a crude extrapolation to slightly different sizes....
Feedpoint Gap size should be about 1.25-inch, although it isn't critical....

Since the Gain curve is dead flat across the FM Band (2.15 dBi +/- 0.1 dB)
and SWR is low, there is no real need to "tweak" for a particular frequency
(as you SHOULD do using twinlead). But if you want, RESCALE both
dimensions SMALLER by the factor 98/103=0.95.

PS: Any Hi-VHF antenna can be rescaled for FM (and vice versa) by
RESCALING all dimensions (including wire sizes) by factor 196/98=2.0.
Gain & SWR performance curves remain the same, after rescaling
frequencies on the bottom axis by the inverse of that same factor.
[Which is a formal way of saying antennas get BIGGER on LOWER freqs.]

BTW: If you use the on-line Yagi calculator, ignore the dimensions for
the Driven Dipole....you have a Folded Dipole, which is a different size.
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post #3890 of 4912 Old 04-22-2010, 11:00 AM
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BTW: If you use the on-line Yagi calculator, ignore the dimensions for the Driven Dipole....you have a Folded Dipole, which is a different size.

The free downloadable Yagi Calculator by John Drew is for DL6WU type folded dipole yagis.
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post #3891 of 4912 Old 04-22-2010, 01:00 PM
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So for 88-108 MHz there are two different size varieties? They seem quite a bit different in size. Are there any mechanical/electrical differences? 55"x 4" seem like quite a different shape/size than 53" x 8." Is there any one that performs better? Are thier SWRs and gain patterns similar?

I've got a length of 1/2" copper, 4 elbows and 2 caps, and ready to build it. Just want your opinions on which to build before I proceed. Thanks Holl_ands.
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post #3892 of 4912 Old 04-22-2010, 02:59 PM
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They're about the SAME, although the 8-in one has about 0.05 dB more Gain
and an infinitesimally lower SWR.....

When all four tubes are SAME size, it forms a Square Loop with 1.1 dB more Gain,
which you'll also find an example for Hi-VHF on my website.
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My next objective will likely be to then add this dipole to an array of directors and reflectors in a yagi format

If youre going to add directors and reflectors to it later, go for the skinnier 55.0 x 4.0 inch format.
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post #3894 of 4912 Old 04-22-2010, 11:10 PM
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Holl_ands, I wish I'd seen your post before proceeding, but I got home early today, and went right to work. I built the 55" x 4" dipole, and probably would have built the other 8" x 53" one, but It's already 99% done. Murphy's Law. Oh, well. I may yet, also build the 8" x 53" variation, as well, but for now, I'm committed to this one.

It was very tricky to assemble, but I went to extensive pains to ensure all the copper and fittings were sanded well (inside and out) for good solder joints, and used silver solder and flux for a good electrical connection.

What I have so far, is a 53" x 4-1/2" (4" center to center) rectangle, cut on one end at 27-1/2" (dead center) on one end. Once I assembled it and cut the one side, it was difficult to get the two sides where the gap is to align properly, as each side seemed to wanted to go it's own way! Anyway, once I was able to get everything straightened out, it occurrred to me to wait and ask your advice before cutting a little off each side for the gap at the feed points.

I remember you stating that the feedpoint gap should be about 1.25". Would that be the distance between the two end caps, or between the two screws, center to center, for attaching the 300 ohm twin-lead?

-What should be the approximate distance between the two end caps?

-What should be the distance between the two bolts (center to center) for the 300 ohm connection point? ( I plan on drilling the end caps and installing #10 brass screws, washers and nuts to avoid galvanic corrosion, as I have on my other antennas for TV.)

I was planning on mounting this on a 1" PVC frame and mast in the form of a "T", reinforced with a wooden broomstick to avoid any complications that might skew the results by clamping it to a steel pipe, such as EMT conduit.

-Would it be acceptable to drill holes through the copper pipe to secure it to a plastic pipe frame, like perhaps in the center of the two 4" ends, or will this also possible skew results, as well?

When I started out, 1/2 of the copper I started with was nearly completely black. Not satisfied with starting with blackened pipe, I decided to shine it up like new with sand paper.- Not just the areas to be soldered, but the whole thing. (I love the appearance of shiny copper!)

In light of my copper "fetish," I would like to ask a question that may sound rhetorical, as I know this topic has been an elaborate discussion regarding TV antennas and reception here before, but I can't help but ask: Will a coating such as paint affect FM performance in the same way it affects UHF/VHF TV signal and degrage antenna performance? I know there seems to be, in this forum, a consensus to suggest any kind of coating will kill the gain on a TV antenna, such as one of Mclapp's 4-bays or a GH, but I'm wondering how it might affect an FM antenna.

I am a just little bit "superstitious," as I know many here are, but I am wondering if a clear coating such as urethane would have a similiar effect as paint, as it is essentially paint without pigment. A spray on clear coat would prevent oxidizing, keeping it it's shiny natural copper color, but I don't want to do it if it in any way, impedes an FM antenna's performance. Of course, If I were to do it, I certainly would not coat the area where the 300 ohm connection would attach.

Copper pipe, shined up nicely and new has a really stealth appearance, and I can imagine it up on the roof all glossy, as opposed to the ugly black appearance that forms over time. I also full well realize that the copper oxide that forms on the surface aids as a protectant from corrosion. I've got to think that even that may also impede any antenna's performance to some degree, nearly as much as a paint coating would.

Many thanks for your continued support and help in this venture.

Best Regards,
Eric
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Perhaps in light of 300 ohm's comment regarding adding reflectors and irectors later, this was the better one to build, anyway. I do anticipate experimenting with this as a directional antenna eventually, and that was one reason I opted for the narrower size, to avoid having to build more, big, fat elements later. Thanks for confirming that for me, 300 ohm. Now I don't feel so bad about building the one I chose.

Best Regards,
Eric
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post #3896 of 4912 Old 04-23-2010, 12:17 AM
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Holl_ands, I am looking at photos and illustrations on your webpage, and I noticed how the builder in one of the photos drilled a hole on the flat side of the end caps on the inside, where the gap is to attach the feed lines. I was planning on drilling on the side of the caps, but I guess I could do it either way. Is this critical, or imperitive to use this method, or does it matter? Could it be done either way, or is the method in the photo recommended ?

Either way, I would like to know if I decide to drill the side of the caps, how far apart each cap should be, and how far apart the bolts for attaching the 300 ohm line should be, center to center, or if I should use the prescribed method of the builder in the photo, and if so, would that 1.25" apply. Thanks again.
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post #3897 of 4912 Old 04-23-2010, 09:05 AM
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You're probably worrying a LITTLE too much....1.25-in is metal-to-metal gap,
which in my Hi-VHF Folded Dipole example would be the end of the screws
that were attached via a hole in the end-caps and then soldered for a
secure, leak-proof connection.

You could just as easily attach the end-caps so that they were separated
by 1.25-in and drill into the sides of the end-caps for the Balun attachments,
but it seems to me that it might be difficult to attach a screw sideways
and difficult to get a good solder joint to the curve inside the copper cap.
PS: There is perhaps +/- 0.5-inch of leeway in the Gap anyway....

I'm no expert re how best to apply protective coatings to bare copper,
but don't worry about it affecting the antenna's performance other than
at the Balun connection point. I recommend brass screws to prevent corrosion.

Since FM F-D is considerably longer than Hi-VHF F-D, it's going to flop around in the
wind if you don't provide some means for securing the floppy ends when you mount it.
I would use PVC to interconnect the floppy ends with the long tube.
I'm philosophically adverse to using wood mounting boards outdoors, since they
absorb moisture, which affects the antenna and need to be repainted every so often....

There are probably a number of ways to do this, but the first thing that comes to mind
is to cut four pieces of 3/4-in Sked40 PVC to about 2-in long each, which are inserted into
a pair of "TEE" fittings. "Notch" ends to fit the copper tubing and finish with a PVC "CAP".
Cut another piece of 3/4-in Sked40 to interconnect the "TEE" spacers. You can put another
"TEE" fitting in the middle, perhaps leaving the antenna skewed slightly off vertical so
that the Balun isn't hitting the metal mast. Glue it all together only when finished.

The copper tubing is going to rattle around in the PVC fitting a bit, so consider using
a small piece of rubber....and/or some metal/PVC friendly glue.

I could also envision a low-budget approach using some plastic pipe clamps
bolted into a pair of 6-in long PVC pipes.....although I have a preference for overkill....
BTW: Copper tubing isn't exactly 1/2-in diameter....YMMV....

An alternative mount is to use a short BOOM (like a Yagi), mounting the long tube
directly to the boom via screws, rivets, whatever...with the Balun mounted to the bottom.
Since the middle is at zero potential, it's okay to connect directly to a grounded mast.
But it will still need some non-conductive support for the floppy ends,
such as the above PVC structure, except without the middle TEE.

Code:
    CAP         CAP
-----|-----------|----- Long Tube
    TEE---TEE---TEE
-----|---  |  ---|----- Floppy Tubes
    CAP    |    CAP
          MAST

Side View:

CAP
  \\
  TEE
   |\\
   | CAP
   |   \\
   |   Balun
   M     |
   A     |
   S
   T
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post #3898 of 4912 Old 04-24-2010, 11:48 AM
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Well, it's the weekend, so I'm home, and going to work on this today. Going to try to get this thing up on the roof. Will post pictures and results.
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post #3899 of 4912 Old 04-24-2010, 03:08 PM
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OK. Here it is. It performs about as good as the twin lead dipole I had up there before it, and doesn't seem to be very directional on any of the FM radio stations I tried it on. I turned it in all directions possible in slight incremental movements, sweeping it back and forth, and noticed no significant change. Locked it up where it seemed to perform best.

Next ambition will be to add it to a boom, and fabricate some sort of a directional yagi with reflectors and directors.

Attachment 173865 Attachment 173866 Attachment 173867
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post #3900 of 4912 Old 04-24-2010, 03:11 PM
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Here's two more photos.

Attachment 173869
Attachment 173870
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