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Re: LF: Re: Re: Antenna Tuning

To: [email protected]
Subject: Re: LF: Re: Re: Antenna Tuning
From: Scott Tilley <[email protected]>
Date: Tue, 04 Jul 2006 20:35:59 -0700
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J.

The key to success in tuning an unknown LF antenna or any for that matter is first finding where it is resonant.

I use a large stable inductor. I call mine the 'mother of all inductors' and it is wound on a big pail and works out to be about 7.5mH. It's not pretty or eff. but will resonant the system somewhere low and has lots of taps all up and down it...

1) connect said inductor in series with the proposed antenna. Use all the inductance... 2) attach a small TX WITHOUT any filtering or other frequency sensitive networks on the output see: http://www.mlecmn.net/~lyle/test-tx/test-tx.htm , use a signal generator as the exciter with a sweepable dial knob or other suitable source as in the link. The key being you need to SWEEP the frequency to find resonance where ever it may be... 3) power her all up at LOW power and sweep the frequency and determine where resonance by monitoring the DC input current. Start low and work your way up until you get the first peak in current. Be careful, you're looking for the first large peak, harmonics above the natural freq can be misleading. 4) measure the inductance somehow (meter, scope, etc...) of the coil and using the known frequency of resonance calculate the C of the antenna. 5) calculate the L the coil should be to resonant that antenna using the C above and 137KHz. 6) build coil with variometer and install, Use scopematch to fine tune the system for resonance and impedance...
7) Enjoy being QRO.

73 Scott





J. Allen wrote:
Alan, Steve and All,

Thanks for the feedback.

The antenna is basically an inverted-L antenna. Using approximate dimensions, the vertical portion is a 110 foot tower, but the attachment point for the horizontal is down at about 100 feet. The horizontal is composed of two, 390 foot long wires that form a V with the included angle of about 50 degrees. The ends of the L wires are supported at about 70 feet and 63 feet and the wires are single span and have bout 10 or 12 feet of sag for stress. The antenna is over a set of about 30, #11 radials of various lengths from 30 to 1200 feet in length. The soil is very dry sand in a flat in the bottom bowl formed by mountains. The antenna is fed through about 250 feet of 50 Ohm hard line from the shack.

At the base of the antenna, the vertical portion terminates at the input of a Variometer the other end of which ties to the radial ground system. The variometer is currently link coupled to the coaxial cable, but the problem was there when the grounded side of the variometer was tapped up like an autotransformer, and also when the antenna was fed through a very large roller inductor / autotransformer. In fact there have been a number of different feed methods tried and all failed.

If I am not mistaken, the antenna calculated at about 1500 pF.

Nothing I have tried to date has resulted in successful tuning. The most power I have been able to sustain is about 30 Watts. Higher power has always resulted in smoke.

The main limiting factor in all this work is the operator... I have symptoms of early Alzheimer's Disease, and get confused about what I am doing or where I am in a process. A written step by step which I could check off as I go would be a great asset.

It is important to the Canadian and possible future US amateur plans for a LF portion on 137 that I be successful here. We have great resistance from the power companies with their Power Line Carrier systems here. I am fortunate that I live in one of the two areas of Canada where 137 kHz is used for PLC, and the central purpose of my experiments is to get the station up to full power and show compatibility of PLC and amateur radio.

I have had really wonderful support from Steve and Scott, and have both of the UK LF books as guides but still cannot seem to get the antenna tuned. I am familiar with the use of an oscilloscope and maintained the PLC system which I am demonstrating non-interference with.

As soon as I can get the antenna tuned properly, I can run the power level up and begin beaconing essentially 24/7 for the remaining months of our experiments.

Has anyone produced a very detailed first time tuning guide for beginners like me? I am about 1000 miles away from the closest assistance (VE7SL/VE7TIL)

Thanks again,

JA
VY1JA






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