Custom UHF Yagi antenna build

[davenn]

. I can’t find guidance on how the 50Ω SMA leads of the VNA should connect to the 75Ω coax.

just use 50 Ohm coax, you shouldnt be using 75 Ohm anyway
you are using a 4 : 1 BALUN .... at most the feedpoint is only going to be around 200 Ohms,
not 300 Ohms
a folded dipoler on it's own has a 300 Ohm feedpoint, but once put into a yagi with other elements,
the feedpoint impedance is going to drop substantially

ohhh and of course, the 4:1 BAULN should be of 50 Ohm coax as well

 

[Guineafowl]

just use 50 Ohm coax, you shouldnt be using 75 Ohm anyway
you are using a 4 : 1 BALUN .... at most the feedpoint is only going to be around 200 Ohms,
not 300 Ohms
a folded dipoler on it's own has a 300 Ohm feedpoint, but once put into a yagi with other elements,
the feedpoint impedance is going to drop substantially

ohhh and of course, the 4:1 BAULN should be of 50 Ohm coax as well

The balun was designed by the software, based on that particular Yagi design and with RG6 specified as the cable. Maybe it’s a compromise, to get the impedances close rather than matching?

By using 50 ohm cable, am I not just pushing the problem down the line? The amplifier and TV socket are 75 ohm, so at some point I’ll need to make the transition.

I might be approaching this the wrong way, but my question would be: you have that crudely built antenna/balun in one hand, and a nanoVNA in the other. How do you connect them together in order to analyse and adjust the antenna/balun to a 75 ohm input impedance?

 

[Guineafowl]

To add to the above:
I’m going to have to admit that the ARRL book is a bit beyond me. It’s a vast collection of knowledge, written by lots of very clever, very technical people. It’s a reference book, not a teaching book.

For example, the first chapter, ‘Antenna Basics’, talks of SWR without first defining it. That sets the tone for the level of assumed knowledge.

In short, what I could do with, is the book you read before that one.

 

[Baluncore]

For example, the first chapter, ‘Antenna Basics’, talks of SWR without first defining it. That sets the tone for the level of assumed knowledge.

Only a few amateurs really understand SWR. The rest of them minimise it, because that is what it says to do in the operator's manual.
https://en.wikipedia.org/wiki/Standing_wave_ratio

 

[Guineafowl]

Only a few amateurs really understand SWR. The rest of them minimise it, because that is what it says to do in the operator's manual.
https://en.wikipedia.org/wiki/Standing_wave_ratio

Yes, it’s easy to enough to understand on a basic level, as the reading on the dial to minimise, but what I mean is, the initialism ‘SWR’ is not first expanded. In a chapter called ‘Basics’. Just one example, but that, and other things, are telling me I’ve skipped over the introductory course.

 

[Baluncore]

The ARRL Radio Amateur's Handbook, is probably preliminary reading for the ARRL Antenna Handbook. There is also the RSGB Radio Communications Handbook.

Amateurs will hear the term SWR while studying for their first licence exam.

It is hard to define an introductory course, because people can arrive from so many different directions. That is where Wikipedia now comes to the fore.

 

[Averagesupernova]

Maybe it's just me but I noticed 35 years ago the ARRL publications were a bit 'scattered' concerning skill level. In the same book they will describe something in very beginner's terms but the next chapter a different subject is way over the heads of many readers. Multiple books with overlapping subjects might get the newby educated if one sticks to ARRL publications.

 

[Guineafowl]

I take it I need a 75 - 50 ohm adapter, one end like this:

And the other a male SMA.

The antenna was put to use today - repairing a 433 MHz car key fob and testing the buttons. I rigged up the antenna to a simple TV signal meter (just LEDs from 50-80 dBuV) and fired the fob at it. Worked a treat.

 

[Averagesupernova]

I take it I need a 75 - 50 ohm adapter, one end like this:
View attachment 345558


And the other a male SMA.

The antenna was put to use today - repairing a 433 MHz car key fob and testing the buttons. I rigged up the antenna to a simple TV signal meter (just LEDs from 40-80 dBuV) and fired the fob at it. Worked a treat.

That is likely just a resistive pad to eliminate reflections on transmission lines. It is certainly not lossless or even close. My guess is 5.7 db of insertion loss.

 

[Guineafowl]

That is likely just a resistive pad to eliminate reflections on transmission lines. It is certainly not lossless or even close. My guess is 5.7 db of insertion loss.

As you can see, I’m a bit stuck. I’m not getting an answer to this question:

I might be approaching this the wrong way, but my question would be: you have that crudely built antenna/balun in one hand, and a nanoVNA in the other. How do you connect them together in order to analyse and adjust the antenna/balun to a 75 ohm input impedance?

Not sure if it’s the wrong question, or if there’s a tacit feeling that I should find out for myself. I would like to get the antenna up and running, but at the moment, the TV signal strength meter is not registering anything.

Another point made to me by a TV engineer on another forum, is that for digital TV, signal strength need only be above a certain threshold, after which point the bit error rate becomes the important factor. The BER is increased by reflections.

 

[Averagesupernova]

Can't have it both ways. A resistive pad will match the impedance at the cost of insertion loss. This may not matter depending on the circumstances. If you are evaluating the performance of one antenna vs another, etc, then just realize that you are burning up power in the matching pad. If you are after maximum sensitivity for receiving weak signals then you will not want to use anything resistive.

 

[Tom.G]

Another option for the antenna is an amplified antenna. (USD$ 50 - 100 here)

A friend has one and gets over 100 stations -- which is pretty good with the TV and antenna in the basement, 30 to 40 miles from the transmitters, and a large hill in the way!

The antenna inself is enclosed in a plastic case roughly 1 foot (30 cm) square and 1 inch (2.5cm) thick. The amplifier is in a small box that connects to the TV.

Last week, during the Solar Storms, he had poor reception and asked for help when getting only 16 stations.

No joy.

The following day he told me he got it working again! He had changed which power outlet the antenna was plugged in to. Originally it was plugged into a 15 foot (5M) extension cord from a distant outlet, and he had plugged it into the same outlet as the TV.

He went back to using the extension cord and all the stations came back.

UHF radio is Weird! (and probably no RF filtering on the power cord)

Cheers,
Tom

p.s. maybe you could get a powered antenna and use the amplifier for your Yagi.

 

[Guineafowl]

Thanks - yes, the Yagi will get a standard amp like this:

I’d like to get a reasonable signal BEFORE amplification, so the next stage, I think, is to use the VNA to optimise the antenna/balun., assuming I’m aiming for 75 ohm impedance at the centre frequency. Should I just hook them together, ignoring the 50-75 ohm conflict, and show you the Smith chart/SWR data?

 

[Averagesupernova]

Since you have a VNA and are interested in getting a proper match, you should have a look at some YouTube videos about using a VNA. I have seen at least one that explains how to very very easily build LC matching networks with the aid of a VNA. Now be prepared for more difficulty at UHF frequencies compared to lower. Start out matching something in the HF band to get your feet wet. You could possibly build something like this on a couple of connectors.