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Low Power AM ATU question....

Posted: Fri Dec 01, 2017 2:35 pm
by waveguide
Hi folks,
I'm going on vacation to the UK in January and I'm doing some work for a friend who is running a low power medium wave/ AM station at a hospital.
He's tried to fine match the antenna (base loaded 7m vertical) with a MJF Versatuner Deluxe II 949E but the ATU is making no improvement what so ever. Are these ATU's capable of matching antennas at this frequency range (AM band)? Is there any modifications I can make to the MFJ 949E to allow me to use it below its specified limit of 1.8Mhz (the bottom of the 160m band).


I've attached a link to where the manual can be downloaded (PDF link).... Thre's a schematic at the end of it.
http://www.mfjenterprises.com/support.p ... d=MFJ-949E

Thanks!

Re: Low Power AM ATU question....

Posted: Fri Dec 01, 2017 3:40 pm
by Bill DeFelice
Might be better off trying to locate a used LPB TCU-20/ATU-30 or a Radio Systems CP-15 carrier current antenna coupler, which have been used with low power free radiate AM systems for campus-limited broadcasting.

Re: Low Power AM ATU question....

Posted: Fri Dec 01, 2017 5:44 pm
by kkiddkkidd
Depending on the load, it _might_ work.

However a 7m base loaded vertical on any AM/MW frequency is going to be VERY, VERY narrow bandwidth and probably very reactive at a very low base resistance. If he is using a Part 15 type antenna that can be tuned to something close to resonance by careful length adjustments, it may still be a long ways away from any resistance/reactance that a 949e will tune. Using the term "tune" very loosely... The feed line may be radiating as much energy as the actual antenna...

He needs to get that very short base loaded vertical adjusted as closely to resonance as possible before trying to patch the match with an external tuner.

Regards,
waveguide wrote:
Fri Dec 01, 2017 2:35 pm
Hi folks,
I'm going on vacation to the UK in January and I'm doing some work for a friend who is running a low power medium wave/ AM station at a hospital.
He's tried to fine match the antenna (base loaded 7m vertical) with a MJF Versatuner Deluxe II 949E but the ATU is making no improvement what so ever. Are these ATU's capable of matching antennas at this frequency range (AM band)? Is there any modifications I can make to the MFJ 949E to allow me to use it below its specified limit of 1.8Mhz (the bottom of the 160m band).


I've attached a link to where the manual can be downloaded (PDF link).... Thre's a schematic at the end of it.
http://www.mfjenterprises.com/support.p ... d=MFJ-949E

Thanks!

Re: Low Power AM ATU question....

Posted: Sat Dec 02, 2017 7:25 am
by Dale H. Cook
If he's running a 7m antenna it is not a Part 15 antenna, as that is limited to 3m with ground lead. He is in the UK, anyway, so not bound by Part 15.

I will second the suggestion of Bill DeFelice to use something like an LPB ATU-30, which is designed to handle a 30 Watt transmitter and can provide a decent load to the transmitter when driving anything from a bedspring to a wet noodle.

Re: Low Power AM ATU question....

Posted: Sat Dec 02, 2017 12:36 pm
by Deep Thought
"base loaded 7m vertical"

As noted above, unless this antenna is tuned very close to resonance you'll never get it to work right. Even if you manage a "match" without doing that using an ATU any environmental change will cause issues. I've fought this battle with TIS antennas here in the USA and it is not for the easily frustrated.

You also need to make sure it has some sort of ground radial system and not just a rod driven in the ground.