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 Zetagi matcher and an aerial

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PostSubject: Zetagi matcher and an aerial   Zetagi matcher and an aerial Icon_minitimeThu May 13, 2021 9:23 pm

Sometimes I think "If only I knew more, or even how, let alone why". Embarassed

1/2 wave pole. Good at high bands, £$%$ on low.
SWR meter found the problem, a REALLY terrible case of 'a bandwidth of 25Mhz up to 30 Mhz' according to the manufacturer REALLY meant:- 'We were only kidding Laughing Laughing!'

I like the really low bands (down 3 blocks in iron age talk) but do you think I could SWR the damn thing in? NWIH could I get it under 2:1

Mid block up to the 'haughty bands' perfect.
Low, Super Low, and you really shouldn't be here Low band,  ARGH!

So, I bought a Zetagi matcher, cabled it in and sat back thinking 'Now what!'
(One of those 'senior moments).

If in doubt see YouTube.
Okay, I now remember the theory (which I did know many moons ago) and watched all the knob twiddlers drop their SWR.

However even the long winded presenters (one went on for 43 minutes) didn't explain which knob to turn first and what the SWR meter was telling you as you gently turned this and that.

So, could someone PLEASE tell me what is the "Best practice" when using one.
What you twiddle with first and the clues the SWR meter gives you about what's going on.

p.s. I really miss my 4mm mains wire inverted H under the gable ends.
There again, I also miss being able to climb a ladder!
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PostSubject: Re: Zetagi matcher and an aerial   Zetagi matcher and an aerial Icon_minitimeFri May 14, 2021 12:27 am

A Zetagi matcher?  Not seen one in person so I did some research.

They all do 26 to 28 MHz, which I feel is kind of strange as typically an antenna tuner will do from 1 to 30 MHz.  This limited bandwidth and where it is suggests that this antenna tuner was specifically intended for CB use, which is odd to me as typically for a single band, any other method tends to be better than using an antenna tuner...

After searching around, I found some pictures of the inside of various zetagi matchers, and I figured out why they have such a small bandwidth.  They don't have any form of variable coil, the one in these tuners is fixed.  Mind you, this is often the most expensive component in an antenna tuner so this is likely why the price is so low.

What these appear to be is a standard T network with an un-adjustable inductor.

To tune these you want the two knobs you have set to the center of their ranges.  Set the SWR meter and leave it set to show reflected power.  Start adjusting the "tune" knob and watch the SWR meter, you want the needle to get as close to the low SWR reading as possible.  Then adjust the other tuning knob for the same.

NOTE:  Once the needle moves you are not looking at the actual SWR, and to see this you will need to readjust the forward reading.  It is possible that you might have to do this once or twice while making adjustments to get to the lowest SWR.

Suggestion, get a two needle SWR meter and run it before the antenna tuner.  Once you are used to it, it is much easier to tune the device with a two needle SWR meter.  I'm surprised that Zetagi doesn't offer this as an option, but when using an antenna tuner, it really is the way to go.

Also, if you intend to run this antenna tuner very far outside of the CB band, get a tuner that has the tuning range needed and use that.


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PostSubject: Re: Zetagi matcher and an aerial   Zetagi matcher and an aerial Icon_minitimeFri May 14, 2021 8:17 am

Hi Paul,


All the Zetagi matchers actually utilise a 'Pi tank' or 'Pi Network' configuration.

That is - First variable capacitor, (Tune), from radio input to ground - across an inductor to the next variable capacitor, 'Load' which is connected between ground and the antenna connector.

It's just like the 'tank' final on many an old valve transmitter that would've been labeled 'Plate' and 'Load'. (See link below for configuration and mention of 'Dipping the Final')

https://www.edn.com/pi-network-and-dipping-the-final/

Best just to look at the first diagram on that link than absorb yourself in the underlying maths. Wink


Another good link to look at is here :-

https://www.dj0ip.de/antenna-matchboxes/asymmetrical-matchboxes/

Ah, that's better, far less maths.


The reason for this layout is so that the variable capacitor controls are wired such that they are grounded. That way they don't suffer the effects of 'hand capacitance' whenever you go to tune them or require extensive, (and added expense), methods of isolating the capacitors from the chassis and front controls.

The reason for one coil or inductor is that they were designed for CB use and not atypical Amateur band HF useage. Why pay for a switched inductor and associated switch gear when you're only using it on one band?


You can tune them as has been previously suggested but it can all become a bit of a pain in the ass chasing your SWR all over the place whilst constantly adjusting your calibration or 'Set'. Indeed, a dual meter setup can be a bit easier in that you aren't flicking your 'FWD/REV' switch constantly back and forth but you'll still be chasing that SWR.


The simplest way to tune them is to just 'listen'.....

Find a frequency that you're interested in but without an ongoing transmission. 

Ensure your squelch is off and select a sideband mode popping up the volume to listen to that lovely cosmic background radiation 'shhhhh'.

Adjust the 'Tune' control until the loudest 'shhhh' is heard, you may have to turn back and forth to find the centre.

Then move onto the 'Load' control and do the same, back and forth until the loudest 'shhhh' is heard.

You're now pretty much bang on where you want to be and can now 'CAL/SET' your SWR meter for an accurate reading.
Then you can 'twiddle' gently a little further, 'tune' first then 'load' whilst reading the SWR meter and often get that elusive 1:1 SWR!


That'll be the settings for that particular frequency, on that particular radio, with your particular antenna. 
Change any of them and you have to retune but you'll usually be just changing frequency not your radio and antenna! Wink

You'll soon find the 'sweet spot' settings for any of the frequencies and take note of where the controls are set. 
As the variable capacitors normally adjust across a 180 degree of motion I use the simple 9 o'clock to 3 o'clock analogy.


What a pain hey? 
Just remember that they are an 'impedance matching device' or Antenna Matching Unit not a 'tuner', they match your radios impedance to the antennas impedance. Now you can see why they don't have simple 'frequency' markings and some don't have markings at all! (That's where the 9 o'clock, 3 o'clock thing comes in handy. Wink) The settings will be completely different for everyone depending on their impedances of their particular setup.


Now, getting onto your 'Lo, Lo-Lo' etc. tuning woes......

You've got a 'Pi - tank' on your radios Finals, matching your radio output to 50 ohm and also acting as a low pass filter.

You have your Zetagi matcher in line which is also a 'Pi - tank'.

You have an End Fed Half Wave antenna, this bugger will have a typical L-C-L base loading coil with a tap capacitor.

.....you may find it difficult to tune.

By all means, give it a go, you may strike lucky but you might have problems with it not reaching that far. 
If that's the case you can simply switch another capacitor in line with the Zetagi matchers first variable capacitor running it in parallel, (remembering capacitance adds when in parallel). A switch squeezed in, placing a good Silver Mica capacitor between the rig side input and ground would do the job. Switch that capacitor in for lower frequencies and switch it out for higher.

You could 'squeeze' the inductor coil between the capacitors to raise it's inductance therefore lowering the frequency but you'll lose the higher frequencies. A longer coil with a switch arrangement to short out the extra length for high/low would also work but requires much more fiddling. But I wouldn't worry.....

...the big problem will be the base loading coil in your half wave antenna, it's a LCL 'T' match to tame the end fed impedance. If you 'clip' out that capacitor your Zetagi Matcher will take over the job of matching. Again, extensive surgery of your antenna and ruining it for anything else except your setup but will allow you to tune those lower frequencies.


In depth I know, but hopefully explains it all a little for you.

The much maligned and misunderstood, but in fact simple piece of equipment any radio operator can have.
So good I built my own, (a Pi - tank no less but with switched inductance), and it may not have 'pretty lights' or do anything automatically but I wouldn't be without it. Smile


All the best,
Victor

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PostSubject: Re: Zetagi matcher and an aerial   Zetagi matcher and an aerial Icon_minitimeFri May 14, 2021 8:39 am

Just a thought when they say 25 to 30 MHz coverage it will do that range but what they don't say is if you alter the aerial length itself.(or rings on the a99)...

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PostSubject: Re: Zetagi matcher and an aerial   Zetagi matcher and an aerial Icon_minitimeFri May 14, 2021 9:17 am

You're not wrong there Alan. They should say 'Tuning Range' not flipping 'Bandwidth'! Mad
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PostSubject: Re: Zetagi matcher and an aerial   Zetagi matcher and an aerial Icon_minitime

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