Custom UHF Yagi antenna build

[Guineafowl]

TL;DR Summary

Trying to improve on a mass-produced TV antenna to solve poor signal.

As discussed with Baluncore, I’d like to make a start on building a Yagi customised to my local transmitter. Here are some details:

Transmitter: Rosemarkie (https://ukfree.tv/transmitters/tv/Rosemarkie). Horizontal. 33km away.

Aerial: inside roof (slate). Landlord won’t allow an external one. At least it simplifies weatherproofing.

Mux frequencies wanted: PSB 1,2,3 at 666.0, 618.2, 642.0 MHz.

I was thinking of a folded dipole with two directors and one reflector, tuned to the middle of those frequencies. At his early stage it seems this should have an impedance of $300\Omega$, requiring a 4:1 balun for the $75\Omega$ coax feedline to the amplifier.

I already have some 1” square aluminium box section for the body, and 3/8” copper pipe for the elements. Various mounts can be 3D printed. I’ll pause here and read the ARRL book chapters on dipoles and UHF antennas, but any thoughts welcomed at this stage.

 

[Vanadium 50]

Questions

1. what are you hoping to accomplish by building and not buying?
2. Do you have a feeling of how much improvement you need over what you have now? 3dB? 30 dB?
3. Do you have interference issues? Does it come from one place?

 

[berkeman]

Landlord won’t allow an external one.

Do you happen to have a balcony that faces that direction? Or maybe a window with a windowsill for planters?

Disguising antennas can be fun...

 

[Vanadium 50]

Disguising antennas can be fun.

Is not antenna. Is modern art.

 

[Guineafowl]

Do you happen to have a balcony that faces that direction? Or maybe a window with a windowsill for planters?

Disguising antennas can be fun...

At some point, I’ll be moving to my own place, so being in the roofspace is a temporary restriction.

what are you hoping to accomplish by building and not buying?

Improve on the presumed compromises of mass manufacture, and have the antenna custom tuned to the local bands.

Do you have a feeling of how much improvement you need over what you have now? 3dB? 30 dB?

No - anything is better than nothing.

Do you have interference issues? Does it come from one place?

Not sure. The TV does glitch when the pressure-jet boiler fires up, but that’s tolerable. We are quite a long way from the transmitter, in hilly country, so the signal is just poor.

 

[Averagesupernova]

I built what is known as a quagi for amateur television some years ago. It is I believe about 15 elements set up for 430 some odd MHz. Plans came from an ARRL handbook. Had one reflector, one driven element and the rest directors. The reason it's called a quagi is because the driven element is a square loop like a quad antenna. No matching, just connect coax to the loop. I see no reason it couldn't be scaled to the band of your choice.
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Edit: the reflector is also a loop.
Plans are in the 17th edition of the ARRL antenna book.

 

[berkeman]

The TV does glitch when the pressure-jet boiler fires up

What's a pressure-jet boiler?

 

[DaveE]

Plans came from an ARRL handbook.

Honestly, you're unlikely to do better than the stuff in the ARRL publications. Antenna design is a bit of theory and a bit of black arts. So many people (except me) have trodden this path before. If you want a good antenna, steal someone else's design (modified for your band). PF isn't the best HAM forum, I think.

If you want to do pure DIY (is there such a thing?), then study EM more and experiment a lot.

 

[Guineafowl]

What's a pressure-jet boiler?

This sort of thing, a Rayburn cast iron range. Pressure-jet is the more modern, forced-air oil mist burner type, as distinct from the older ones which used a wick. Common in rural kitchens throughout the country:

It’s either the oil pump\fan, solenoids or spark igniter that glitches the TV.

I don’t intend to reinvent the wheel, but if I can adapt an existing design to the local transmitter frequencies, and get an improvement, that would be great.

 

[Averagesupernova]

Antenna design is a bit of theory and a bit of black arts.

I can't completely disagree but I think the black art part is because in the past, the software tools that we have now did not exist. Admittedly I have not used any of these tools myself. I have looked at the theory and designs of simple working antennas and more or less wrapped my head around it.
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The first and most common place people get confused over antennas is that it usually appears the feed line is driving a short or an open.
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You are correct in that the ARRL is an excellent resource that's hard to beat. Might want to think twice about modifying a design for a different band if you're taking a two meter 15 element beam and trying to scale it to 40 meters.

 

[Baluncore]

The RF signals you give are centred on 666.0, 618.2, 642.0 MHz.
Check those numbers, and arrange them in order.
Will more channels be added to that site later? If so, include them now in the plan.
618.2 MHz = chan 39. Was 614 to 622 MHz.
642.0 MHz = chan 42. Was 638 to 646 MHz.
666.0 MHz = chan 45 Was 662 to 670 MHz.
How wide is each digital TV RF channel? Maybe 8 MHz? That makes each ±4 MHz.
Lowest frequency is; 614. MHz.
Highest frequency is; 670. MHz.
Geometric mean is; √(614*670) = 641.4 MHz.
(The average of the wavelength and of the frequency is the GM).
So you need to design your Yagi centred on 641.4 MHz.
It must operate without problems between 614 MHz and 670 MHz.

You will have a reflector and a driven element, plus several directors.
Too many directors, and you will miss the high and low frequencies.
Too few directors and your directivity gain will fall.

Now is the time to start finding and playing with free Yagi design software.
What is available these days?

 

[Averagesupernova]

Now is the time to start finding and playing with free Yagi design software.
What is available these days?

Now I'm curious as well. Definitely be watching for replies here.

 

[tech99]

I think it is highly unlikely that you will obtain better gain than a commercial antenna having, say, 10 elements. It is true that manufacturers throw away some gain to obtain wide band operation, but even so, the design and construction of a Yagi for these frequencies is very critical. Even the diameter of the boom and the method of mounting the elements have to be taken into account. So I don't think your own 4 element narrow band Yagi will beat a commercial wideband 10 element one. Nevertheless, it is an interesting field, so I should give it a try.

 

[sophiecentaur]

The TV does glitch when the pressure-jet boiler fires up,

You could try checking thre atual RFI level from the boiler. Perhaps an additional RFI filter n the mains supply.

 

[Baluncore]

So I don't think your own 4 element narrow band Yagi will beat a commercial wideband 10 element one.

With my antenna designer's hard-hat on, I know that a tailored Rosemarkie specific Yagi, will outperform a broad-band commercial compromise, something that was optimised to sell, and to maximise profit, throughout the country.
For UHF, the DTV Yagi will have a few more than 4 elements.

 

[Vanadium 50]

I still don't understand the buy vs. build logic. Yes, one could probably design something for your specific need that improves on what is available commercially. But probably not for the same money. One can buy a decent but not great attic antenna and amplifier for about $65.

Trying to get a solution with some idea of the target is tough (which is why I asked). If you need a little more gain than an indoor antenna that's one thing. If there's a mountain between you and the transmitter, that's quite another.

If one is getting RFI, just about the hardest place to attack it is at the receiving antenna.

 

[berkeman]

I still don't understand the buy vs. build logic.

From his previous thread about building a different antenna:

TL;DR Summary: While working through the ARRL antenna book, I’d like to start putting some theory into practice.

So he is combining leaning practical antenna construction with building antennas that he can use himself. Seems like a good goal to me.

BTW, does anybody have a feel for how much gain there could be to having 3 seperate Yagis (one tuned for each frequency) versus the wider band version that covers all 3 bands? Could it be as high as 3-6dB? If so, the 4x higher cost might be worth it (4x to include the coax switch).

 

[Averagesupernova]

I'll be blunt. I just don't get the buy instead of build crowd. No, I don't build my vehicles from scratch. I don't build power supplies and test equipment on my bench from scratch. But seriously, how far do we take this? Why amateur radio at all? Just pick up the phone. Building things after learning the theory behind it teaches more than the theory. If it doesn't, you weren't paying attention. A first time build of an antenna will often reveal things that could be considered at least partial failures and demonstrates there is room for improvement. This is learning and I never consider learning a waste of time.

 

[Vanadium 50]

BTW, does anybody have a feel for how much gain there could be to having 3 seperate Yagis

Gain? Or gain in the gain?

The end frequencies are less than 4% different than the center. So I would say "almost none". The wavelength difference is under an inch.

 

[berkeman]

Gain? Or gain in the gain?

LOL, improvement in the RX level.

The end frequencies are less than 4% different than the center. So I would say "almost none".

Yeah, I suspect you are right.

 

[Baluncore]

BTW, does anybody have a feel for how much gain there could be to having 3 seperate Yagis (one tuned for each frequency) versus the wider band version that covers all 3 bands? Could it be as high as 3-6dB?

If the Yagis are not kept far apart, they will couple, and distort patterns. In general, when you double the length of a Yagi, you halve the bandwidth, but add less than 3 dB to the gain. A longer, narrowband Yagi, is also more critical to manufacture.

In itself, narrow bandwidth has no real advantage, unless you suffer cross-modulation, or de-sensing, from an out-of-band source, but then a deep notch filter would be more applicable.

It is better to have two short Yagis in a phased array, (+3 dB), than one of twice the length.
That becomes more important as the wavelength falls from VHF to UHF and the effective energy capture area, λ², of the antenna becomes less. The logical evolution is a cake-tray backed, phased dipole array, that has width and height, not length. It is easier to trade vertical and horizontal directivity, and improve signal strength, with a 2D array, than it is by making a longer Yagi. The cake-tray reflector F/B ratio is also improved over a Yagi.

 

[sophiecentaur]

I don't build power supplies and test equipment on my bench from scratch.

Moreover, you may well be in a position to actually measure the performance of a lot of those toys, yet you buy them. Antennae are an act of faith in many respects because, unless you have an open field (i.e. grass) and a mast with a rotator etc., you really can't tell the performance. Make it or buy it, you have that same problem and DIY can be more expensive than buying poorly constructed commercial domestic ones; DIYers tend not to have the sort of machine tools needed. for short-cuts. Manufacturers will have measurement capabilities (but do they use them?)

But we have to admit that learning by making is the essence of Amateur Radio. If someone really wants to to build antennae then that's absolutely fair enough but, as with boat building and guitar makeing etc. the project can get seriously in the way of using.

I would make the point that many of the beliefs about antenna function and performance are untested (objectively and in detail). I haven't mentioned HiFi yet; 'nuff said.

 

[Baluncore]

I would make the point that many of the beliefs about antenna function and performance are untested (objectively and in detail).

What would some of those beliefs be ?
Are those beliefs held by amateurs, or by professionals ?

 

[Vanadium 50]

This has been bugging me. Isn't 642 MHz UHF, not VHF?

 

[Baluncore]

This has been bugging me. Isn't 642 MHz UHF, not VHF?

Yes, it is UHF.

 

[Averagesupernova]

If someone really wants to to build antennae then that's absolutely fair enough but, as with boat building and guitar makeing etc. the project can get seriously in the way of using.

Yes it can. What if the enjoyment comes more from the building than the using?

 

[sophiecentaur]

What would some of those beliefs be ?
Are those beliefs held by amateurs, or by professionals ?

Many amateurs and many contributors to 'the technical press'. There's no need to be defensive for this because we know it doesn't apply to you but we all know that the harder it is to do actual measurements, the higher the level of BS and anecdote. If basic calculations were good enough then why do professional installations cost more than amateurs can afford?
Simulations have improved a lot in the past decade so perhaps the home antenna problem may have been cracked.

Yes it can. What if the enjoyment comes more from the building than the using?

Oh yes. Many briliant musicians couldn't hope to make a good instrument but the maker can get a lot of fulfillment from it. However, the user may well be in the hands of the dodgy shopkeeper's advertising blurb. HiFi is probably the best example.

 

[Averagesupernova]

However, the user may well be in the hands of the dodgy shopkeeper's advertising blurb. HiFi is probably the best example.

Ok, this gets to the center of discussion of which is better than the other. Two store bought products, two hand built, one store bought and one hand built, etc.
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If the guy playing the guitar and anyone around can't tell the difference, then what's the problem? Somehow it's more wrong for a guy who built a one off antenna/guitar/hi-fi amp to throw a bunch of BS to potential buyers as compared to anyone selling store bought stuff? I once heard a guy selling mobile 2 meter magnetic mount antennas say they were better than a mechanically mounted antenna because there was magnetic coupling to the ground plane (body of vehicle) compared to mechanically mounted which in time will developed poor connections. Nothing involved here was homemade except the salesman's ridiculous statement.

 

[berkeman]

I think we should stick to the original topic of the thread, which was helping GF with his home-built antenna. Thanks.

 

[Baluncore]

The first step is to design an approximate prototype, from λ/2 ±5% elements, spaced λ/4. For long, high-gain narrow-band Yagis, use DL6WU, or a modern derivative. (Not this case).

Search 'Yagi design software free'.

Then run something like the old DOS program YO, to optimise F/B and pattern gain at the centre and boundary frequencies. Get an idea for the way things trade. Change the number of directors to get the bandwidth required, while keeping the best gain possible across the band. Play it like a video game. Maybe there is a free copy of YO here.
https://www.iw5edi.com/software/yagi-optimizer
Warning: YO ignored element resistance, so could explode out of reality, into purely fictional, imaginary designs.

 

[darth boozer]

Log periodic antennas are generally preferred for wideband operation, while Yagis are essentially narrowband.

https://k3rrr.com/diy-log-periodic-dipole-600-700-mhz-tv-antenna-not-coat-hanger/

 

[Baluncore]

Log periodic antennas are generally preferred for wideband operation, ...

The Log-Periodic Dipole Array was originally analysed and described as being a series of 3 element Yagis. It turns out that, when you look closely, significant RF currents only flow in two adjacent elements, which makes the LPDA worse than a 3 element Yagi. The advantage the LPDA has, is in the monitoring or surveillance of entire bands, when an LPDA is mounted on a rotator at the top of a tower.

... while Yagis are essentially narrowband.

Three element Yagis are not narrowband. The bandwidth of a Yagi is dependent on the number of elements. The maximum gain is about 16 dB. Beyond about 24 elements, there is no gain advantage, just more wind buffeting and ice damage.

The LPDA is only used for Digital TV when the transmit site RF frequencies cannot be first identified, such as when the antenna is sold nation-wide by a chain store.
For DTV, if you know the RF frequencies, a short four element Yagi will outperform an LPDA.

 

[Baluncore]

Attached is a prototype 6 element YO file, RM_6.YAG as text, with element dimensions. More than 6 elements starts to limit bandwidth or gain.
The performance of the prototype is here.

 

[sophiecentaur]

For DTV, if you know the RF frequencies, a short four element Yagi will outperform an LPDA.

It depends a lot on your requirements and one should always start from there. Looking up at all the antennae on neighbouring houses can be helpful in choice of what to buy or make.

I live a long way from a main transmitter and the multiplexes sit over a very broad range of frequencies and powers. So I'm starved of received signal strength here and there are no multi-path generators nearby. That was the major factor in my choice of a twin yagi array. Someone near the transmitter with an office block nearby would have totally different requirements.

Designing and building is a long job and the only basis for accurately judging performance is A/B comparison against the last one you put up. But construction can be fun and you end up with a cupboard full of bits that are suitable for marks 4,5 and 6 models. 3D printing is a godsend.

At least antenna building is a daytime sport so you still have viewing time in the evening.

 

[Guineafowl]

I’ve made a first attempt with a designer:

This is Yagi calculator by VK5DJ, v.2.6.20.
3.15mm is the diameter of my welding rods, which I have plenty of. Coax not selected as it doesn’t have RG-6.

And the performance - not quite as good as Bauncore’s: