How to build a UHF antenna... - Page 28

Thanks very much to everyone here for all the great info.

Here's my blog post about the beefed up youtube-style-coathanger antenna I just finished:

--erm, go to blargification.com (wait for the refresh) and click the top blog post since I'm too much of a n00b to be allowed to post links--


It's 11" bowties, about 5/8" feedline gap (going by 6*diameter, it's all 14ga wire), made onto a 2x6, so the spacing between the feedlines and the reflector is 5.5". Eventually I'll get a better reflector than the al/cardboard/half-roll-of-duct-tape one, and I might upgrade the balun eventually (though I have no problems with this one).



Interesting, however, is that I get 17 at ~70% signal (according to the pcHDTV-5500 in my MythTV box) but 16, which is on the same physical tower and a few kW stronger, comes in at ~60% signal.


Anywho, anyone see any minor tweaks I can do other than move it outside?

PsychoI3oy,
That's a major indoor antenna, and everything looks pretty good. I can't see much you can do to make it much better. The coated wire will change the resonance of the antenna but I don't know how much. It changes the velocity factor of the wire some which may shift the frequency curve but shouldn't effect gain within the curve.

I've been doing some further modeling on reflector spacings and have found that even for the lower UHF channels that 4" spacing is slightly better, actually spacings near 3" give best raw gain. The wider reflector spacing like 5 1/2" gives a slightly better impedence match but the narrower 4" spacing gives better gain which gives a slightly better net. The difference is probably not worth the hassle since you already have it built.

I have a test stand built for some semi-controlled outside testing and reflector spacing is one of the tests I'm going to be doing.

Looks like you had fun and learned alot

Cool, thanks.

I could always go over it with a razor and strip all the wire in place, but that'd probably result in more bent elements than I care to straighten.

As for the spacing.... Yeah, I don't think I can shrink that down without having to rip down the length of the 2x6.

If anything, I may end up increasing the spacing after Feb because of the two stations going back to 7 and 9 to see if it helps with VHF-hi gain.

I'll be testing the 15" spacing soon. I'm going to modify a 10" 4 bay reflector so I can slide it in and out to test spacing and it's effects on net gain. I plan on testing in the 3 - 6" range and 14- 16" range, both UHF and VHF-HI.

Neat idea!
Sort of an adjustable 4-bay, eh?

It would be nice to gather the available information on this thread, and organize it into a nice "4-bay FAQ"/"Build guide".
(Maybe as a sticky?)
Then people starting on their first build could really customize their antenna to their available channels/needs.

Just a thought...

I posted this on the HDTV tech forum I see this maybe the place to get the answer.

OK, I see the elements are 7" long in a v shape with 4 V's down the board 6" apart at the screw's, Element tips are all 3" apart.
Maybe 8" elements 5" apart.
I made mine with 3/4" steel strapping.
Are these the best measurements?
I can make longer elements and adjust the spacing just give me the measurements.
Please give me the "A","B" and "C" measurements if you have a better
4 element design like mine with no reflector.
How far apart should each of the V's be spread at the tips at the A
and how far apart should each element tip be from the next element tip B down the board ?
I hope that is clear what I'm asking.:helpme
Also does it matter how far apart the elements are across from each other, C ?
As in the left row of screws distance from the right row of screws.
Thanks in advance, Paul

Yes, strictly for test purposes so I can confirm some of the antenna modeling that's been done while I have use of a spectrum analyzer.

The info is getting pretty spread out on this thread when it comes to the 4 bays. I was thinking of making a detailed page of my own once I confirm some of my computer modeling and currently built antennas. I'll be sure to post my findings here as well.

So far I've just done some quick tests with some folded dipoles to use for reference and various home made antennas looking at the gain curves and peak gain. It appears that the models are very close to the actual test site readings. The 10" whisker 4 bay with the curved reflector and forward swept elements is between 10 -14db over the dipoles on UHF channels 14 - 50 with a peak gain in the channel 32 range. It falls off real fast after channel 52.

Much more to come on these test in the next few weeks weather permitting.

From what I've learned so far the "A" dimension is not real critical especially when using wide flat elements like yours, 3" is probably good enough. The dimension from connection point to connection point and the distance from tip of element to tip of element are the critical measurments. It appears that it's better to have a closer "C" dimension and longer elements than a wider "C" dimension and shorter elements.

I've never used V elements that wide so I don't have a good feel for how long they should be but generally the fatter the element the shorter the length they need to be.

If I were to make an educated guess I would say make the V elements (metal strapping) about 9 1/2" long from connection point to the end. The vertical distance between the V elements at the connection points (phase lines)should be 10". The "C" dimension should be 1 1/4".
It may be best to strip all the insulation off of the wire. Leaving the insulation on will change the way the signal travels through the phase lines.
I don't know how much it would change the velocity factor but if the insulation is off that will get rid of an unknown.

Those dimensions should get you coverage of channels Ch14 - 50.

Leave the strapping flat on the outer ends and taper them down on the connection point end, pretty much the opposite of what you have now.
It won't make much difference but should improve band-width some and be less dangerous in case it falls over it won't stab the cat to death.

If you want to favor the higher channels cut the dimensions in proportion a little shorter and to favor the lower channels cut a little longer.

Good luck

Thanks for the quick reply, I will try building one to the spec's you gave me.
If it fell 2 wienerdogs would be the victims and I would not want that.
I will take some signal readings before and after and post you back on how it works.
Thanks again,
Paul

Yeah, thats one vicious looking bow-tie. Very medieval, heh.



Mclapp,
Ive noticed from the Ken Nists models, that he models large flat surfaces like that using two parallel wires, in other words, both the edges of the single piece. I thought Id mentioned that in case you wanted to model it.

Ive got a large stock of flat chrome plated aluminum about an inch wide with an 1/8 lip on them for stiffness, but I havent quite decided what to make from it.

My first thought when I saw that picture was, "hey, that's a neat way to recycle old garden shears!"

I've modeled some stuff that way, that's how I ran the model of the winegard style 4 bay I built which uses flat elements instead of V's. I guess when I test it we'll see how accurate modeling it that way is.

How did that thing work out just in rough guess testing ?

I just had it up for a quick test and looked at the overall curve and relative gain. It's doesn't have the band width of the whisker bow tie but it's plenty enough to cover 14 - 50.

The gain is about 1-2 db down on UHF from the 10" curved reflector 4 bay bowtie antenna but it only has a 9" element length and spacing so on the lower part of the band it's going to come up short on gain.

What I want to test is the VHF-HI performance, I have one at my friends house and he's been picking up CH11 @ 85mi. since we put it up in the spring. I thought for sure when the leaves came on it would lose reception but he's had it all summer.

The weather looks good this week so I should be able to get a large amount of testing in by the weekend, too bad it's getting dark by 7:30 now.

A couple of weeks ago I downloaded the files for a 4-bay array with 9.5" whiskers and 9" spacing by mclapp. I was wondering if there is any data for gain versus frequency, and swr. Thanks for all the simulations you guys do.

Also, has anyone tried MMANA-GAL simulation software. It's a free download. A friend at work uses it a lot for HF ham antennas and gets very good correlation between his simulations and measured data. He's a Phd RF engineer with lots of RF design experience (hombrewed his own moonbounce station). The software includes sample files for HF and VHF/UHF ham band antennas. I've been trying my hand at modifying the sample files for different frequencies but nothing from scratch yet. Google "MMANA-GAL" and it will take you to the download website, if your interested.

Thanks for all the excellent information!

JM WB4OFT

Tell your friend about 4nec2, also a free download (and is free-ware) with many samples, I think he'll be impressed. From what I understand, the MANA programs are cut down versions of the NEC engine, developed for use on PC's when PC's didnt have anywhere near the computing power they have now. They are accurate but of course, have limitations. The free 4nec2 program I use (and mclapp's non-free EZNEC program) use the full blown NEC2 engine developed by Lawerence Livermore Labs for use by the Navy on a Cray 1 supercomputer. But your friend better have a computer with some horsepower, this is a killer app. Modeling something like a 16 bay antenna with a full screen grid with 8000 or so segments/patches and 2000 pattern lines takes like 12 or so hours on my 2.8 ghz Core2 duo processor. The program uses both processor cores. It could also be used as a good cpu burn-in program or a benchmark program, heh.

Do you want the info for the 4 bay array with or without reflector and if you want the data with the reflector do you want flat or angled/curved.
In my recent field testing I have found that the frequency curve for these antennas is actually lower than they model.

Here are the UHF plots for just the 9 1/2 x 9" array no reflector.




Attached is a text file with the pattern analysis and impedance data

 

4bay 9-5x9norefl.txt 84.12109375k . file

I saw two of these antennas yesterday when driving in a rural area,about 50 miles from the towers. Any idea what company makes these?

Thank you 300ohm for the info about 4nec2 free-ware version. I had not heard of it. I'm going to download and check it out.
JM

Thank you very much mclapp for the antenna data. That was exactly what I was looking for . . the array only - no reflector.

If you want to buy one thats very similiar, the Phillips sdv9011k_17 fits the bill.


Also download the extras, like the files that increase the number of segments to 11,000 or so, and the 3D viewer option. Thats a really a really nice viewer.

Also there is a fairly large learning curve with it (think about the learning curve you first went thru when learning spreadsheet software) but dont get discouraged. Download and read the 4 part beginners guides, they are very good. Then go thru the examples in the _GetStarted.txt file.

You can get them at Lowe's. They're Philips something or the other (MANT901 I think is one of the models.)

But don't even bother with these. They are very flimsy and don't perform well at all.

I bought one, tried it, and returned it when I got 0 DTV stations from NYC, with the antenna on top of my 70ft (86ft with the mast) tower.

The design is a typical UHF corner reflector/yagi and a VHF LPDA (log periodic) combo.

They even claim 100 miles with it, which isn't really the case.

I put up a CM4228 and I'm much much happier with the results.

Mclapp,

Thanks for the drawings (pdf files) showing how to build a four bay (9" x 9.5") capable of pulling in lower UHF channels. They made it very clear and easy to make my own.

Best of all, it worked wonders!

The Seattle Seahawks are broadcast on channel 18 which is nearly 40 miles away with several hills in between. I first made the 7" x 7" as mentioned in this threads' OP. It pulled in the closer stations really well, but only occasional blips of ch. 18, and only if pointed just right.

Now, using your 9" x 9.5" design channel 18 is rock solid over a 15 degree swing -- without a reflector!

Another plus is very good analog VHF reception for channels 9, 11 & 13, which in our area will go digital after the transition.

Thanks again!


FYI for everyone: I used 12 guage galvanized electric fence wire (just as is) for both the phase lines and wiskers. My son has a big spool he uses to make chain mail.

I'm building an indoor set-top antenna with elements made from #10 copper wire. Should I coat the wire with something to prevent tarnishing? If I don't coat the wires, roughly how much would I lose in gain?

Thanks,
Jean

No need to worry the only place that tarnishing will make a difference is at the element connections and if the connections are tight that won't be a problem either, especially indoors.

The effect of elements attached to wood and contact area have been discussed on many occasions. Over the weekend I did some testing and one of the antennas I tested was a 9 1/2" x 9" 4 bay mounted on a wood frame. It's the same one that I posted pictures of many posts back, it's just a piece of dry pine with wood spacer blocks coated with urethane. The spacer blocks are about 1" wide so the phase lines are in contact with it for nearly an inch. The elements and phase lines were made of #12 copper wires. When I tested this antenna it was about .5 - 3 db down from the other similar antennas tested.

I removed the phase lines and elements from the wooden frame and attached them to a ½ PVC frame and retested the antenna. It gained .5 - 2db overall and the gain peak moved up 50 MHz. I don't know for sure if it was due to the wood or the larger contact area the elements and phase lines had with the wood compared to the PVC or both but it definitely made a difference.

The area where the elements attach to the phase lines is considered a high voltage area relatively speaking and is the most sensitive to contact with other objects. The high voltage area changes with frequency but is mainly centered around the attachment area. The best solution would be to avoid any contact with anything in this area but that makes construction difficult.

The 9 ¾ 4 bay shown in the chart has custom plastic spacer blocks to minimize the contact area and that one performs the best overall.



Also in this test I trimmed the whiskers of the 9 1/2" x 9" to 9"x9" and tested a hoverman and a classic 6 1/4" closed end bowtie.

A series of 4 folded dipoles ranging from 6 1/2" to 11 1/2" were used as reference to get dbd readings.

The test were done by mounting a transmitting dipole and the test antennas about 20ft. above ground and spaced just far enough apart to minimize ground reflections. A B&K 2630 spectrum analyzer was used for the measurements.

I did alot of other tests this weekend as well and I will post that data later but I thought this was the most important of all the stuff I learned or confirmed.

Well... Armed with mclapp's new info, I think I will wrestle my homebrew 4-bay out of the attic this week! (darn hardware cloth reflector has to be flexed a bit to get through the attic trapdoor...) I'll have to make a visit to Menard's/Lowe's/Home Depot to see if they have any plastic pieces (blocks, washers, stubby pipe pieces, etc...) that I can fashion into standoffs. Maybe I'll simply pick up a big bag of vinyl washers.

I'd like to thank you again, mclapp, for all your hard work!
You're gathering some really valuable information for all of us homebrew antenna folks!

I don't know if a couple of db is worth the trouble, but if you do wrestle it down and use some plastic stand offs let us know what you did and if you saw any difference.

I would suggest short pieces of PVC pipe or PVC connectors for stand offs, the round contour will keep contact with the elements and feed line to a minimum and they are cheap.

Also check your yellow pages and see if you have a wholesale or retail plexiglass / lexan dealer in your area. They are a good source of new scrap plastic in many thicknesses for cheap.

I think those are philips brand. I am not 100% though. I do know that the exact same antenna is being sold by menards price is about $55

They look very cheap and the if I wiggled the display, the whole antenna vibrated quite a bit. Could just be the build or the guys who put it together, but I wouldn't want it.