Low Power AM ATU question....

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waveguide
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Low Power AM ATU question....

Post by waveguide » 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!

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Bill DeFelice
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Re: Low Power AM ATU question....

Post by Bill DeFelice » Fri Dec 01, 2017 3:40 pm

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.
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kkiddkkidd
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Re: Low Power AM ATU question....

Post by kkiddkkidd » Fri Dec 01, 2017 5:44 pm

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!
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

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Dale H. Cook
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Re: Low Power AM ATU question....

Post by Dale H. Cook » Sat Dec 02, 2017 7:25 am

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.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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Deep Thought
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Re: Low Power AM ATU question....

Post by Deep Thought » Sat Dec 02, 2017 12:36 pm

"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.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

W2XJ
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Re: Low Power AM ATU question....

Post by W2XJ » Fri Jan 05, 2018 11:24 am

I agree with most of the above but I would approach it this way. First model the antenna, EZNEC should be adequate and then when you are there, measure it with the actual ground and other enviromental elements. You can then design a network and see if the MFJ has enough range although I seriously doubt it. Alternately just remove the reactance with a simple series element. This will yield a very low mostly non-reactive impedance which can be brought to something usable with either a transformer or the correct length transmission line which will most likely be near a multiple of a quarter wave. I used this approach with a 50 KW DA on one potentially troublesome tower that was about .1 ohms. It has been stable over 30 years.

Another trick that is used by some very reputable engineers is put a non-inductive resistor in series with the tower and bring the R to some usable value and then match. The problem is greatly increased loss which could be overcome with increased power if UK law permits. Also, depending on local regulation, you need to measure current going into the actual radiator and drive the system with enough power to overcome losses. This would depended on if authorities permit an EIRP determination or if you are limited by transmitter power. Good luck!

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Re: Low Power AM ATU question....

Post by R. Fry » Fri Jan 12, 2018 6:59 am

Below are the results of a spreadsheet based on the opening post, and the assumptions shown in the graphic.

When this defined system is tuned to resonance, the sum of the radiation resistance, matching coil resistance, and the connection to a "typical" r-f ground for such systems can result in a feedpoint impedance fairly close to 50 Ω (in this case, 55.581 ±j0 Ω, or 1.1:1 SWR).

The only tuning device needed is a loading coil at the base of the monopole with enough reactance to offset the capacitive reactance of this short monopole at the AM carrier frequency. A coil of about 122 µH would do that, in this example.

As mentioned here earlier, the r-f bandwidth of this antenna system will be narrow. The antenna system would be fairly difficult to tune up, and remain tuned up when installation conditions varied for any reason.

Disclaimer: The legal use of this system is left to the user to determine.

Image

waveguide
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Re: Low Power AM ATU question....

Post by waveguide » Wed Jan 24, 2018 2:21 pm

Thanks. Ok, so I have managed to tune the base loaded antenna - but as someone said - its incredibly narrowband! Only about 30khz approx.

The resonant sweet spot can vary depending on weather (by about 20khz), so I need to weather proof the house of the loading coil. Hopefully it will be stable then.

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Bill DeFelice
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Re: Low Power AM ATU question....

Post by Bill DeFelice » Thu Jan 25, 2018 6:46 pm

waveguide wrote:
Wed Jan 24, 2018 2:21 pm
Thanks. Ok, so I have managed to tune the base loaded antenna
Did you go with the MFJ tuner or with something else?
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waveguide
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Re: Low Power AM ATU question....

Post by waveguide » Tue Jan 30, 2018 2:47 pm

Hi,
No the MJF just wasnt able to do it. I emailed MFJ and they pretty much said it just won't work, with any antenna, I asked them about modifications (which there are apparently!) - but they couldn't comment or suggest any - invalidate warranty ect.... !!

So trial and error with a base loaded (about 12 turns on a 10cm former with about 7m wire and I got it. But VERY narrow!!!!! :)

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