How to build a UHF antenna... - Page 104

That should be included in the laws of physics. Just like gravity, its gonna happen, and theres nothing you can do to stop it, heh.

Chapter 18, page 16.

But I think the biggest reason they dont really include the bowtie design is that its a very wide band antenna, and you design for a range of frequencies for it to cover, not just one. And there are a lot of tweaks to it. For them to cover the bowtie properly, they would need almost a separate book.

They also dont include the Hovermann, or quite a few other antenna types I can think of. That doesnt mean its a bad book, heh.

Thanks, I agree

Also:
1. The ARRL Antenna Book, Tenth Ed., 1964, p. 246 (shows actual bowtie)
2. Antennas, John D. Kraus, 2nd Ed., section 8-7, pp 354-358
3. Antenna Engineering Handbook, Johnson and Jasik, 2nd Ed., Section 4-3 Biconical Dipoles, pp 4-12 & 4-13;
Section 29-5 Triangular-Dipole Antennas and Fig. 29-5, pp 29-12 & 29-13 (TV reception antennas)
4. TV Antennas and Signal Distribution Systems, M. J. Salvati, 1979, pp 61-66
5. Radio Commnication Handbook, RSGB, 5th Ed., p 12.35 (fan dipole & fullwave conical dipole)
6. http://books.google.com/books?id=QN8...age&q=&f=false
7. http://books.google.com/books?id=6_X...ntenna&f=false
8. The ARRL Antenna Book, 18th Ed., Chapter on VHF and UHF Antenna Systems, section on Collinear Antennas and Large Collinear Arrays and Fig 9 (showing the original 4-bay collinear array that is the basis for the 4-bay bowtie antenna that is popular on this thread), pp 18-8 and 18-9.

The 4-bay collinear array they show in Fig 9 consists of 8 halfwave elements arranged in 4 pairs of collinear elements (or 2 halfwaves in phase) with halfwave vertical spacing between the stacked bays and looks something like this:



This collinear array, just as it is, is very broadband and forgiving of construction errors. But, in order to cover the whole UHF TV band it is necessary to modify the elements for even more bandwidth and make the design frequency and element length for the center of the required band. The most popular method is to use V shape "whiskers," but it is also possible to use large diameter tubing, wide flat strips, or solid triangles.

[quote=rabbit73;17167392]Also:

6. http://books.google.com/books?id=QN8...age&q=&f=false

I had fun looking at this. A blast from the past. I can't decide if I should smoke Lucky cigarettes, or Prince Albert. The articles and ads are remarkable, well worth looking at.

I have a question, though, and I need to preface it: I am not an engineer, and am also not versed in antenna theory. I just want to make something that works.

Now, it just so happens that I made a video for Youtube which more than 12,000 people have watched, and it grows by a few hundred daily. I get a lot of questions from people regarding that video, I kid you not. I answer all of them.

I can help most people with their questions because I've read all the posts in this thread, but once in a while I need to use intuition. I hate it when I don't have solid answers, though.

The latest question I couldn't answer with authority had to do with combining 2 4 bay antennas using splitters. I advised the guy that one properly made 4 bay would probably work better than 2 4 bays connected by splitters. My answer was that splitters aren't a good choice for combining antennas. I also advised him to look at some commercial designs for ideas.

What do you think? Did I get it right?

Thanks

Thats tough to say, a lot variables involved. Some over the the DHC forums have reported better reception combining antennas (CM4221s) with a splitter. But that could also be not from increased gain, but a change in the pattern. With a splitter, you have the loss for the splitter plus the losses of the two baluns (and not counting the insertion/connection losses and cable loss)
For double bay antennas, youre much better off building mclapps M8 or a DBGH with the phasing lines. Then you only have to deal with one balun.

Most splitters work well as a combiner and can be used with very little loss in the combiner. The trick comes in getting the the proper positioning of the 2 antennas so that they work together and getting the phasing right.

The main key with the phasing is that both antennas are fed with exactly the same length of coax and the baluns are both identical and phased properly. If all that is done right you should get 1 -3 db improvement depending on the channel.

A splitter when used as a combiner will only have about .5 -1db of loss by itself, the reason why the loss is more when used as a splitter is because you are cutting the signal in half at each port (3db) plus insertion losses of the .5 - 1db mentioned above.

The best way as 300 ohm mentioned, is a well designed and built open wire phase line but the combiner/splitter set-up is simpler and still works well.

Hey Everyone-

I am looking to build my first UHF antenna based on mclapp's design. I am using 12ga drop ceiling wire for the elements and a 1x4 for my mounting post. I have a few questions before I get started...

//TVFool
VHF-HI = 7 & 9
UHF = 15-50

Based on the above & the attached TVFool.png, I am gathering that I need to fab 8.5" whiskers.

- Can I get suggestions for dimensions to use for the phase line? (I did go through 104 pages!)
- Do you still recommend a .5" separation (bending/twisting) at the phase line cross or just electrical tape or shrink tubing?
- Should I use standoffs from the mounting post to allow for the spacing at the phase line cross?

Thank you in advance!

So, from what you and 300 are saying I was mostly wrong. Still, that's a lot of explanation for an almost complete newbie, which this guy is. Also, on the cm4228 they use wires to connect the 2 4bays. I copied that design once, but I admit it only worked marginally well for me.

I have RF7 and 12 in my area so personally I would go with a 10 x 9.5 -- works for me


I believe the general guidelines for the phase line spacing is around 1.25".


I am not sure if that .5" separation amount is cast in concrete. From some of the discussions here it is recommended you have some separation. On the other hand, for my area at least and the frequencies I need to receive (and perhaps just plain dumb luck) I've found it had no noticeable affect with or without separation. Hence my easy-to-build version 2 pvc build with no separation and version 3 aluminum with separation. They perform about the same (see my sig below).

Since you said you are going to use a 1x4 for you spine I would have to recommend you do use standoffs to keep the phase lines away from the 1x4. And if you do that it cost you nothing to space the phase line cross-overs thus adhering to the general guidelines of separated cross-overs.

Will this configuration receive channel 50? My TVFool looks much more depressing than yours and I need every DB I can get. :^)

I would say that without the proper dimensions and guidence that someone would be better off sticking with a 4 bay as you suggested.

As you found combining antennas isn't always as good as you may think. For a newbie the combiner set-up is probably the best way to go but still no where fool proof.

You may want to go with a 9 1/2" x 9" to help with the upper channels, the 10" x 9 1/2" will still receive in the 50's but starts to loose real fast up there. Once you go below a 9" whisker/phaseline the lower VHF-hi channels get degraded quite quickly.

I would suggest also using some sort of plastic stand-offs as Me! suggested.

Mounting the elements and phase lines directly to the wood hurts the gain, especially on the upper UHF channels. Use some seperation at the X over area 1/2" minimum and make sure both wires are the same length.

I understood that I should keep the spacing at 1.25". To clarify, I am looking for the dimensions as to where to put the bends based on 8.5" whiskers. Is there a formula?

Thank you again for the assistance. I anxiously await your replies because my pixelating TV is trying to teach me some new hybrid form of Klingon-English. ...Klinglish ...Englon ;^)

Sorry mclapp,

I must have been typing when you replied. I will make a 9.5" x 9" and see how she works. Off I go to the HoDo to get more parts.

Thanks again!

Keep in mind, to keep the 1.25" spacing, you want to drill the screw holes further apart, like 1 3/8" as suggested in mclapps plans.

Understood. Will be following exact spacing instructions as per the two PDFs.

One question: I read somewhere in this thread to add 1/2" to the cross to account for bending of the phase line. Would the measurement be 2.5" or 3" (bowing to 2.5") at the cross?

Making whiskers in the mean time.

02Ocean224,

Here are mclapp's diagrams for the 10x9.5 4-bay. They work very well!

Plans for the 9.5 by 9 mclapps are here: http://www.frontiernet.net/~mclapp/A.../diagrams.html

 

4 bay phase line for 10 inch elements.pdf 32.853515625k . file

 

4 bay 10 inch flat elements.pdf 21.7255859375k . file

I'm glad that you enjoyed the references. Some of the old ideas are still good and have withstood the test of time.

Your Youtube video is a big improvement over the previous videos, especially the ones that claim that 7" whiskers and vertical spacing of the bays is optimum.

I actually made signal level measurements at my location comparing a 4221 with a 4228. The 4228 gave one channel that was 1 dB stronger than the 4221, most were about the same, and a few were a little weaker than the 4221.
The main advantage of the 4228 is that its horizontal beamwidth (because of its horizontal stacking) is narrower than the 4221 which helps with multipath problems (the signal strength might not be better, but the signal quality ((BER)) is improved, which helps the tuner maintain lock).
I also found that which balun is used can make a difference with certain channels. In my case, a Magnavox outdoor balun was better than the CM outdoor balun for one of my channels.

You gave a good answer.

One well designed 4 bay can be better than 2 4 bays combined with a splitter especially if the smaller antenna is placed in your best location, which is why mclapp has been focusing on designing his 4 bays with the optimum element lengths, larger reflectors, and curved reflectors.

In my case, using a signal level meter, moving my 4221 to a better location gave me more gain than if I had combined FOUR 4221s. If I didn't have a signal level meter, my second choice would be the Apex DT502 CECB and my third choice would be the signal level percent scale on a DTV.

Using a splitter to combine two 4 bays is an "iffy" solution. When I combined two 4 bays with a splitter for CH 15 I got 2.5 dB more gain, using baluns that gave the highest readings for CH 15. I didn't, at the time, check the other channels, because a friend needed an antenna optimized for CH 15 (PBS). The point that mclapp made about the two baluns needing to be phased correctly is right. If you end up with much less gain with two 4221s, then reverse one of the baluns.

Tests have been made by others, using better equipment than I have, showing that many factors are involved, like the actual length of the two identical coax lines from the two 4 bay antennas (a null might appear just where you don't want it):
http://www.hdtvtruckee.com/
http://www.hdtvtruckee.com/CombinerMeasurement.htm
www.hdtvtruckee.com/CombinerComparison.htm

So my order of priorities would be (in increasing difficulty):

1. Find the best location for your present antenna. This will often give more gain than combining two identical antennas that are aimed in the same direction, and the smaller antenna is easier to handle.
2. Make sure that your antenna is the best for the channels that you want. If not, modify your DIY antenna or buy a better one.
3. Try AGAIN to find a better location. With UHF sometimes a few feet up or down, left or right, or even forward or backwards (which I noticed when I mounted a 4221 on my car---a lot like when listening to FM in a parked car, it is sometimes necessary to move the car a few feet to get out of a "dead spot" null in the wavefront) can make a big difference.
4. Stack/combine more than one antenna and use any device available to you to measure the improvement. (Can you tell that I'm hung up on measurements?)
5. Give up on your marginal channels.
6. Sign up for limited cable or a dish.
7. Move to a better OTA signal location.

Thanks for the 10" plans, IDRick. My question still remains though. Does the 2.5" measurement at the phase line cross account for the bend required to maintain >=0.5" spacing between them? Mclapp?

...makin' whiskers.

Yes, it does account for the crossover. I have constructed several mclapp 4-bays with a 1" gap at the crossover. I am able to do so with 10" long phase lines for the outer two bays and 9.5" long phase lines for the middle bays.

Good luck with your build!

Rick

The highest frequency around here is RF51, i.e., the highest you can go. But it's a foreign language station I never look at. On the other hand RF50 (a PBS station) I do look at a lot and it comes in great. But it's also only 13 miles away. In my version 3 post I have an
attachment comparing that antenna with my pvc build. So you can see how it perfoms across the band. There is a large dip at RF47 and 49 but they are also lower power stations (and yet more foreign language stations).

The answer I gave was only to reply directly to your question about receiving up to RF50. But, given your 1edge and 2edge situation, you can't go wrong with what mclapp says and go with the 9.5 x 9.


Some folks try to mimic the way the CM4221/4228's do it. But I never could do those bends smoothly like that. I prefer (and it's much easier) to simply cross over the phase lines "naturally" from the inner standoff to the end standoff and space them (if using high enough stnadoffs). That's also the way Antennas Direct DB4/8's do it. I am not sure it changes the performance any which way you do it.

Whichever way you do it I suggest you don't try to pre-cut the phase lines to the requisite length before you mount them. Make them longer and trim them so that they don't stick out past then end standoffs after they are in place. Then they will end up of equal lengths.

Thanks Me!, (Dear Lord, am I talking to myself again!)

I am feeling reassured since I am in the process of building my 9.5 x 9. As to the natural feel, that was my plan all along but wanted to be sure. I do have one final question...

-Does it matter which way the twist goes on the phase wire? on one side do they both go to the back while the other goes front? Or does the twist look more like a DNA strand? Does my question make sense?

...Finished my whiskers, now I am stuck on this little variable.

Yeah, like he concludes, without a lot of expensive equipment, its gonna be a lot testing, moving, retesting, try different baluns/splitters, retesting, moving, retesting, etc, etc, etc.

Correct, one phase line should go under at both top and bottom crossovers and the other phase line should go over at both top and bottom crossovers.

Doesn't really matter much, I like to use the long twist instead of the one in the diagram which is a copy of a 4221. The diagram accounts for the extra wire to do the twist (4221 style) but if you use the longer DNA style twist it will use slightly less wire. Heres a picture of one I built, this is 1 four bay of my 8 bay vertical stack but it shows the longer style twist. I used small plastic spacers at the twist point which are really not needed.

I'm not an expert at antenna reception like McLapp, 300_ohm, or Rabbit, not to mention a bunch of others, but my video may give you some ideas about building if you plan to use a wooden spine.

maximus57 wrote:
Cost = $0 Source = old tv antennas ...
I made the 8th one by a creative method of combining a short piece with a longer one. The creative part was that I used a firecracker to blow one open and then pinched it down on the other one.
-------------------------------------------------------------
That was a terrific idea! Too bad it wasn't suggested before the 4th of July when Chinese (used to be Japanese) "party favors" are available at my local (ultra poor) reservation sites, adjacent to the (ultra rich) gambling centers. Next year for sure!
Jim

I remember some discussion about baluns some time back. It's hard to believe something so simple can make a significant difference.


Well. this all makes me feel better. I do advise finding a good location, but think this advise goes unheeded a lot of the time.


This is something I read in this thread before, but forgot about.


Five and six don't seem hard. Painful, maybe, but not hard.

Thanks

rabbit73 wrote (two quotes):

... The V shape "whisker" elements of the Channel Master 4221 and mclapp's DIY antennas that are arranged in this manner: ><, are called "bowties" because they are a simplification of bowtie shaped elements used in previous antennas.

... The correct term for this >< arrangement of elements is a fullwavelength fan dipole which is a simplification of a fullwavelength conical dipole. All of these various element shapes are used to give more bandwidth than a straight wire to cover the whole UHF TV band.
------------------------------------------------------------
Rabbit, et al:
I purchased a very tired bowtie-driven corner reflector on Craigslist last weekend for experimentation. Mechanically, the antenna is very poorly designed and its trying to fall apart - but the boom-structure is rock-solid. I attached similar photos below (stolen from someone else's post, here).

Per what I have learned in the current thread here, the bowtie shape is meant to expand the antenna's bandwidth - but, what if I wanted to redesign this corner reflector to be centered on a specific frequency, in this case channel 31, my local KONG 16?

Can I change the driven element to a folded dipole, change the length of the reflectors (based on a Yagi) and 'dedicate' this antenna to 31? How would I determine where to position the focalpoint of the driven element?

The devil is in the details: KONG 16's (31) transmitting antenna is side-mounted about half-way up the KING 5 (48) (NBC) tower and it's antenna is still side-mounted just above KONG. KING has been granted a construction permit to changeout their VHF antenna, but I suspect this will happen next year. No clue if KONG will be raised to the top as well (no permit has been applied for) or if it will inheret the current KING antenna's location. Distance from me is 0.8 miles, but I am totally shadowed by the steep hillside and a 35' tall concrete retaining wall holding up the hill, within less than a block from me.

Additional: after constructing two more cut-to-channel Yagis, I now have all three network channels that transmit within a few blocks of each other (and me). As mentioned above, NBC 48 / ABC 38 and CBS 39. Fortunately, the best reception heights for my antennas are several feet apart from each other.

Testing at ALL heights between below my roof level and 24 feet up, I have tried to get 31 with my cut-to 35, 38 and 48 Yagis, a CM-4221, CM-4228, a very strange tiny corner reflector, a strange teeny-tiny periodic, a rather small combo VHF periodic/UHF corner reflector, a horizontally mounted or vertically mounted single 8" diameter loop (from the back of an old TV) BUT! The only time I was able to receive KONG, I used a Winegard HD-9095 (it has about 950 elements-LOL!) mounted well below my roof level. I really don't want to put that monster up, because it will be within a few inches of my property line VS the City of Seattle VS my neighbor! AAAaaack! Thanks in advance!
Jim

PS depending on the antenna I am testing, I have accumulated 33 channels-so far ... hopefully before winter hits, I can get the appropriate antennas up on one mast.

PPS I disassembled my ancient 'standard' low/high VHF antenna and I rebuilt it as a dedicated channel 9 antenna -- guess what -- BINGO! Channel 11, (their tower is next door transmitting at 12.5 kw ERP) and it is invisable. Rats!