ATU Question

Hi All,

If I tune a 25-30 mHz mobile antenna to 28 to 30 mHz, can I use an ATU for 27 mHz?

Thanks 26CT4550

Hi Steve,

You certainly can, although there may be many that say otherwise.

Give it a go and I'm sure you'll be happy but I'll also give an explanation as to why below.
(This is for the benefit of any others reading too. Skip if you're easily bored!)


The most important thing with any sorting of matching solution is accounting for losses. Not so much in the actual ATU or matcher itself, (negligible when tuned 'correctly'), but the fact that the higher SWR due to the antenna now being too short will still be there after the ATU and so will any coax or feed line losses. All you're doing is keeping the radio happy with an impedance match.

The antenna will continue to work as it's still within it's design range but it may alter the radiation pattern a little.

A good (but sometimes contested) explanation is available here :-

https://palomar-engineers.com/non-resonant-vs-resonant-antennas


The actual SWR shouldn't be that far off so any losses in the coax after the tuner to the antenna on a short run will be fairly small.

You can check this with the Coax Loss Calculator available here :-

https://www.qsl.net/co8tw/Coax_Calculator.htm


From this you can deduce why so many hook ATU's up at the antenna end to avoid coax losses on long runs.

A good explanation is available here :-

https://owenduffy.net/antenna/iobr/

Towards the bottom of the page is a great explanation of a well known and common 'non-resonant' antenna i.e. the 5/8ths wave.


Many see 'resonance' (at 50 ohms) as the only 'true' way of using an antenna but often forget about commonly used antennas that usually have an inductor (or winding) within them usually utilising some sort of added capacitance too. Basically an impedance matcher doing the job of what an external ATU would do.

For other antenna solutions feed line losses are alleviated by using a different feeder system. I utilise open line feeder (600 ohm) on my own antenna system meaning that losses are kept to a minimum even though my radio and ATU are in my shack with the antenna being further way. In such a way even though I could have a very high SWR I happily run only one 66 foot long antenna to cover all bands & frequencies from 1.8MHz(160m) through to 10m(28-30MHz). I can always get a SWR of 1:1 and it actually works well too.

Of course even a 50 ohm dummy load will give you a 1:1 SWR across a wide range of frequencies but it doesn't make it a good antenna!

With contacts from the USA to Japan my setup certainly isn't a dummy load.



I've been told before now that, "if ain't a G it's CB", in scorn to my M7 licence yet I've learned the above in pursuit of my own antenna/radio happiness in the short two years I've been licensed. Of course there'll always be some that regardless of years licensed/experienced will disagree with it all.....but then some people firmly believe the world is flat too.


Apologies for my waffle (I'm renowned for it here at CT) and a huge thanks to the few here that steered me on the right path towards antenna understanding.
(Still learning!)


All the best,
Victor

it's good waffle though Victor very educational to read it 

Excellent waffles as always Victor. I read it but it just bounces off the brain . The links are all added to my bookmarks though for future reading, in hope that one day it will start soaking in. 
One day I will be clever....one day

Thank you for a superb and detailed relpy Victor.

Regards

Steve

26CT4550