How to build a UHF antenna... - Page 105

I currently have a 36x36 inch reflector for my mclapp 4-bay. Is there a way that I can block part of the reflector to test varying widths without actually trimming off material? This piece was expensive enough that I don't want to cut it down, just temporarily test smaller widths.

Also, I use carlon straps to mount my phase line and whiskers. The mounting bolts tend to loosen slightly over time. I currently use flat washers. Would lock washers improve the situation? If so, which type should I use: teeth in middle, teeth on outside, or teeth on both middle and outer edges? Can you use a lock washer at the balun connection?

Thanks!

Yes, the bowtie does increase the bandwidth, but I see no advantage in changing to a folded dipole driven element. The bowtie, which is a collinear pair of halfwave elements, has 1.6 dB gain over a folded dipole. And, the top and bottom of the bowtie are bent to conform to the corner reflector shape. Making the bowtie a little longer might help.

The reflector rods of the corner reflector are not resonant like the reflector rods of a yagi, but you usually can increase the gain by making them a little longer and adding a few more to increase the size of the corner reflector.

You could add director rods cut for CH 31 in front of the driven element, but even with all these modifications I'm not convinced you would do any better than the antennas you have already tried. If you do add directors, I favor using the folded dipole as the driven element because it couples better with the parasitic directors. A long yagi designed for CH 31 might do it. A corner reflector that I built didn't do any better than my 4221.

If I were in your situation I would very much want to have a signal level meter to see if I was making any progress.

Considering your difficult location, I think you have done extremely well.

Hi Rick!
I don't know of any way to block it.
Yes, the wide mesh is expensive. You could improvise a smaller reflector with aluminum foil and cardboard but a roll of 1x2 mesh 24" wide and 10ft long isn't very expensive:
http://www.hardwareandtools.com/
http://www.hardwareandtools.com/invt/6929160

Bolts do tend to loosen when holding soft materials. I would use a flat washer, a spring type lock washer to maintain pressure, and a nylon insert locking nut. The nylon will eventually fail in very high temperature (some nylon pulleys in the hot trunk of my car cracked), but there are also lock nuts that have a fiber insert, and there is a type of lock nut that has no insert but is deformed to maintain pressure on the threads. There is also a liquid called Locktite that you can put on the threads to hold the nut in place.

Best regards,
rabbit

Hi Rabbit!

I'm playing with a 2-bay mclapp design for my son and also wanted to compare it to my 4-bay mclapp with 1x2 hardware cloth. Thanks for the links! Last year I could buy a 10' length of 1x2 hardware cloth locally at Lowes for $13 (but didn't). This year they are not stocking it... I can buy it by the foot at the local Cal Ranch but they want $2.25 per foot (36" wide 1x2 hardware cloth), not very good pricing for comparing multiple sized reflectors! Have you compared aluminum foil lined cardboard versus 1x2 hardware cloth? If so, did you have similar gain with both types of reflectors?

I did see some 1/2x1 hardware cloth that was 24 inches wide. Any tips on how to join two pieces together for varying widths (say 28, 32, and 36 inches)?

Thanks for the tip on the lock washer and nylon insert locking nut? Can I use this combination at the balun attachment as well or will this rip up the u-shaped hocks on the balun attachment?

Thanks again and all the best,

Rick

I compared aluminum sheet with 1/4 inch square hardware cloth and didn't measure any difference in gain. Once the vertical spacing of the horizontal wires in the reflector has been reduced to 0.1 wavelength, any further reduction will not produce a measurable increase in gain.

mclapp says it's OK just to overlap it if you fasten them together.

I don't see why not.

Ok, great! I plan fixing the height at 18 inches and compare varying widths. The cheapest route would be to use aluminum foil over cardboard. Alternatively, I could buy 3 ft of the 36 inch wide 1x2 and make varying widths with overlapping and zip ties. That would be easy to do up on the roof.

Best,

Rick

Hi Rick, did you try Home Depot? By me they have 1/2" hardware cloth in 3'x5' for ~$10, not too much better


Yes I see what you mean, it probably will. Use 2 flat washers at the balun connection with the balun "U" hooks between, and the split lock washer under the head of the screw.

BTW, got any pic's of your build?

Hi Hooper!

Thanks for the tips! Great idea for the balun connection! Hmmm, I wonder if the cooling racks from the dollar store would work as a reflector? They might be just the right size or not.... Should be cheap though!

***add in*** I searched this thread and found Hooper is one of our oven rack as reflector experts! Where did you get your oven racks and what were their dimensions? What was the rubber tubing that you used to tie the racks together???

Right now I have a rough test prototype. I'll post pictures when I have finished version. It'll still probably look like it was built by Calvin and Hobbs but, oh well... LOL

Best,

Rick

My pleasure Rick. I imagine those should work just fine! If I remember the maximum vertical distance between the wires on a reflector is around 1" apart to appear as solid to the signal. Keep in mind if the racks aren't wide enough and you need to attach them together, to remove the coating where they touch so you get metal to metal contact, and the finished reflector appears as one piece.


EDIT: Thanks Rick, but I'd hardly call myself an expert!
Actually they're not oven racks, but wire shelves from an old bakers rack I had in the basement. Each shelf is just over a foot wide x 28"lg. and the wires are 1-1/4" apart. The 3 shelves are tied together (and to the PVC frame) with wire (zip) ties, and is 28" x 38" overall. (Not counting the overhang of the bigger rods on the shelves.) I guess I'll add this info to my web page.

Seemed to work well and I saw no difference when I added 1/2" mesh hardware cloth to it. My collinear build definitely seems to get a stronger signal, in my driveway anyhow. Although the G-H calls for a split reflector, so this could be playing a big part.


Very funny Rick!

I got bored waiting for a buddy of mine to come over and give me a hand trimming some trees so I can get my G-H in the air, so I painted it. (May as well go ahead and make up the top hat NAROD's too!) I attached some pic's as well as the orig bakers rack trial.

Looking good.

Thanks 300,
Just as a question do you think there would be a problem if I post these diagrams on my site?

Also looking at the top hat, I do have your dim's correct, yes?
BTW, where exactly is the optimum placement away from the stubs? (0.5"-0.6" is like a 20% variation, seems like a lot to me.)

Thanks

That's a gorgeous antenna Hooper! Very nice work! Thanks for the info my friend!

All the best,

Rick

rabbit73 wrote:

... Making the bowtie a little longer might help.
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?: Is there a formula to determine the length or perhaps the area or the 'best' shape of each bowtie-half, based on its' intended "center" frequency?
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The reflector rods of the corner reflector are not resonant like the reflector rods of a yagi, but you usually can increase the gain by making them a little longer and adding a few more to increase the size of the corner reflector.
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WHY? Are they 'blockers from other directions?' How about if I cut them to a calculated length for a reflector for a 31 Yagi? Then they would be resonant. Is this a good or bad plan? - and why ---
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You could add director rods cut for CH 31 in front of the driven element ...
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May I assume adding front directors would be based on the feedpoint of the bow rather than the forward bend of the bows?
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A long yagi designed for CH 31 might do it. A corner reflector that I built didn't do any better than my 4221.
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Rabbit my friend ... you know I am not opposed to building Yagis! LOL! Almost every local has tried to stop me from building them ("try a paperclip") but so far, they ALWAYS work! What else can I say? However, combining my working antennas will be another issue ...
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If I were in your situation I would very much want to have a signal level meter to see if I was making any progress.
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Yeah I know, but then comes a new scope and a new faster 'puter to chomp thru the data. Speaking of that, I lost my THIRD Western Digital hard drive in less than 18 months, two days ago, and I'm now on my steam-powered 450 mhz backup machine. Maxtor drives from now on.
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Considering your difficult location, I think you have done extremely well.
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Thank you, and I really appreciate your words. None of the local (genuine) engineers on my local list seem to 'get it'. Well, maybe two do. But, that's what THIS forum is all about! Imagining and building solutions.

I had a private mailing (from a member here) regarding this particular issue and I intend to follow thru with his (bizarre) suggestion presented to me. I know this is a "black-art" and if his idea works I will share it here.
Jim

Just wanted to give a quick update on testing with my 2-bay mclapp. I ran a simple test with the antenna on my deck and used my 36x36 inch FLAT reflector with 1x2 hardware cloth. Should be big enough to see optimal response! PLEASE read these results as being specific to my particular test and situation.

VHF

The BAD news:

There was a negative response to the flat reflector when it was spaced 4, 5, and 6 inches behind the antenna elements. Signal strength decreased by 9% on average which equals a 6.3 dB decrease in margin to dropout! (Hat tip to mclapp, he told me to expect a poor response).

The GOOD News

Spacing the flat reflector 16 inches behind the antenna elements returned ch 8 signal strength to normal. I should state that I only have one high vhf station so this effect may be specific to my situation only.

UHF

In my test, there was limited response to the flat reflector on UHF (only 1 to 2% unit improvement in signal strength) at all reflector spacings. At my location, I have consistently seen limited responses to adding a flat reflector with a 4-bay and now with a 2-bay.

NEXT STEP

Evaluate an angled reflector with swept forward elements with the 2-bay and do the test up on the roof.

Hey Rick, thanks for the compliment!

I'd say a 36" x 36" reflector's big enough for a 2-Bay!

From looking at mclapps gain data I'd expect most of what you found - except that you didn't see any improvement on UHF when adding the reflector. By his data it looks like a solid 3dB jump when adding a reflector at 15" behind, and at least another 1.5dB at 4-1/2" back. Of course these numbers are from 4-Bays, but I'd expect similar results from a 2-Bay. I'm sure you know the page I'm referring to.

You should say that a reflector doesn't work for YOU. Could be the materials you used. I've made a 2 bay of mcalpp's antenna( 9.5 in whiskers 9 in spacing ) and I can tell you that a reflector makes nearly 50% improvement in signal strength for me. My reflector was just an oven rack which is 17.5 wide( so it's not even enough to completely cover the width of the whiskers ) and 23.5 inches high.

Well, you make a great point. I have gotten a 2 to 3 dB increase in margin to dropout with an angled reflector and swept forward elements on my 4-bay 10x9.5 mclapp. But, response has always been limited with a flat reflector. Based on mclapp's recommendations, I use a flat reflector and swept back elements to increase beamwidth on the 4-bay since my towers are 70 degrees apart. I'm optimizing beamwidth rather than gain with the 4-bay.

BCF68, can you fill in the details on the 50% improvement in signal strength? Does this mean your signal strength on your tv went from say 40% to 90% or did you see a change from say 50% to 75%? ****Add in, did you see the negative effect on high vhf that mclapp predicted from his modeling and that I observed with my 4, 5, 6 inch spacing tests? If not, I'm curious what is different in your situation?***

You are correct, my original post was not specific enough to my individual test. I will revise it shortly.

Hey Hooper!

Which graph are you referring to in your above comments? I'd like to see it. I've looked through the plots on mclapp's site... Perhaps I missed it.

Best

Rick

All I can tell you is not using a reflector I would for example say have a 30% signal. Put on the reflector and it goes to 45% take the reflector off back down to 30%. That tells me it's 50% increase. Similar results on other frequencies and signal strengths so I know I wasn't getting just a 15% increase. Now on higher signal strengths I obviously didn't get 50% for example if I was getting 70% I didn't go from 70% to 105% for some reason my converter box has topped out at 94% and it rarely reaches that. Usually 92% is the max signal I can get in no matter what. My reflector by the way is 4 1/2 inches behind the bowties.

As far as High VHF. Well in my area we have 2 stations on hi-VHF, 8 and 10 and both are hard to get in using a 4 bay and though possible to get in, not very well on a 2 bay. So I can't really tell you as to whether or not a reflector would improve reception or not on a 2 bay. I don't have the 2 bay to test anymore since I gave it to a friend though I suppose I could make another one.

Thanks BCF68! Good explanantion.

John D. Kraus, who is considered the "father" of the corner reflector, in his Antennas 2nd Ed. gives a design chart for the bowtie in a corner reflector on p 558 which says 4/5 of a wavelength for the total length of the bowtie. If you are interested in corner reflectors you really should borrow this book and read the chapter on reflector antennas. The general idea is that fatter elements don't need to be as long for the same resonant frequency.

In the VHf UHf Manual, 4th Ed., by RSGB there is a chart on p 8-15 for elements using tubing that tells how much shorter to make the collinear pair for different length to diameter ratios: Table 7. Resonant lengths of full-wave dipoles. For example: for a wavelength to diameter ratio of 50 the length would be 0.85 and give a feed impedance of 500 ohms.
Since you would be designing for only one channel the tubing would give the 1.6 dB extra gain over a folded dipole and be easier to calculate. If they are made too long you have less gain because the main lobe divides in two. You can see this in the gain vs frequency curve with 12" whisker bowties; the gain drops like a rock at the high end.

Your design calculations could be verfied with computer modeling. I'm not into computer modeling and have made a choice not to do it and let others do it for me. I'm 76 years old, have been doing antenna experiments since I was 8, and would rather spend the rest of my time building, and making empirical tests and measurements.

Your fondness of the folded dipole, corner reflectors, and directors makes me think that you would be happy with a 91XG UHF antenna; it has a lot of gain and is very popular.

The reflector rods of the corner reflector do indeed block signals from the rear and give an excellent F/B ratio, but their primary function is to reflect the incoming wave front to add it to the direct signal that is being picked up by the driven element which is located at the focal point of the reflector for increased gain. The reflector rods do not need to be resonant any more than the planar reflector wires of the 4221 or the dish of an antenna with a parabolic reflector need to be resonant.
Some hams have tried multiple resonant reflectors and found that they weren't worth the trouble. The case of trigonal reflectors giving 0.75dB gain comes to mind.

yes, feedpoint

So, how DO you tell if a new antenna is better, other than observing how your tuner acts at the "cliff"?

Good luck with the experiments, Jim!

I'm the last person who would tell you not to try something different. Kodachrome film, one of the finest color slide films, was invented by two "outsiders" Mannes and Godowsky who were musicians. The didn't know that it "couldn't be done," so they did it.

Best regards,
rabbit

Hey 300ohm,

Do you have any tips on bending pvc to make a curved reflector? I have a hot air gun for melting plastic crystals (used to make waterways on a model railroad). I'm sure it will get hot enough to bend the pvc!

Thanks!

Rick

Hey Rick,
I actually saw the gain comparison graphs somewhere else...

I didn't want to break any rules by linking to another forum - but I guess you leave me no choice!
http://www.digitalhome.ca/forum/showthread.php?t=96178

So you're a MRRer also?
Those plastic crystals wouldn't be the Woodland Scenics stuff would they?

Hey Hooper,

Sorry my friend. I'll take the fall for ya, okay?! Thanks for the link! Yeah, I have fun model railroading. But, I really need my artistic college age son to do the scenery stuff... LOL!

All the best,

Rick

Lots of info determining performance parameters for Corner Reflectors can be found at
www.cebik.com, including some truly high gain monsters...
Requires FREE registration.....

I think that's www.cebik.com, and while when Mr. Cebik was alive it was a useful site, it appears the new owners have decided to put all the content behind a wall. They are more proprietary about the content than the actual creator was.

Yes, it does seem that way.

L. B. Cebik was a very talented modeler who had an extensive knowledge of antennas. I will miss him.

Fortunately, his work has been collected and preserved for others.

Hi akahooper, you can post them on your own site but there are some steps you need to take to adhere to the GPL license.

First, have a prominently displayed link to this URL with those diagrams:

http://www.digitalhome.ca/ota/superantenna/index.htm

Next, copy and paste the the GPL license text onto the same web page as your diagrams (copy as is, formatting and all)

Yes, that will work excellent. ( Even a hair dryer will work, given enough time per joint, about 10 - 15 minutes).
I use an old electric heater.
Other methods : Sticking the pipe in a running car tail pipe, heh.

Yes, it looks correct, given the .5 inch bends.
Use .55 inch, center of wire to center of wire, for the optimum placement if you can measure that close.