TUNING YOUR ANTENNA

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WALKINGMAN
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TUNING YOUR ANTENNA

Post by WALKINGMAN » Mon Sep 28, 2015 3:26 am

Hi


do you tune your AM unipole antenna to resonance, then match the feed point impedance via ATU. or just match the antenna WITH HIGH REACTANCE until swr 1;1

which is better and realistic of the two.

walkingman

rich wood
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Re: TUNING YOUR ANTENNA

Post by rich wood » Mon Sep 28, 2015 4:28 am

Howdy, I have tuned several monopoles. You set the short to tower leads so the self impedance of the skirt is close to 50 ohms for the frequency you are using. The reactance is going to be pretty high possibly 400 or so. Then put the ATU in circuit and set the input for 50 J 0 at the operating frequency. Make sure all connections in the skirt are tight and coated with an anti rust compound. Also, if there are any other feed-lines on the tower, have grounding kits every 1/16th wave of the AM frequency.

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Deep Thought
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Re: TUNING YOUR ANTENNA

Post by Deep Thought » Mon Sep 28, 2015 7:28 am

If you have an actual matching network (not just a series capacitor) then I've found you'll have fewer problems if you can find a bond point that keeps the reactance below +j300 ohms and the resistance above 25 ohms. As Rich mentioned, often the 50 ohm point will have a reactance above +j400 (sometimes well above), which will lead to instability with weather (particularly icing) and very high voltages at the feed point.
Last edited by Deep Thought on Mon Sep 28, 2015 7:30 am, edited 1 time in total.
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ChuckG
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Re: TUNING YOUR ANTENNA

Post by ChuckG » Mon Sep 28, 2015 6:28 pm

Were you ever able to add an output cap to your tuning network? IIRC you were trying to resolve a reactance problem by adjusting the skirt instead of modifying the tuning network? Is this the same antenna?
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rich wood
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Re: TUNING YOUR ANTENNA

Post by rich wood » Fri Oct 02, 2015 3:52 am

Deep Thought mentions keeping the resistance down closer to 25 ohms to reduce the reactance. I installed a Knott Ltd skirt kit and they insisted that we tune for 50 and just "live with" the high reactance. Setting it lower in impedance sounds like a better plan. The stability of the skirt is fair, but you are right in icing conditions it gets "funny".

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Re: TUNING YOUR ANTENNA

Post by ChuckG » Fri Oct 02, 2015 8:55 pm

Nott still suggests a skirt can compensate for a deteriorated or marginal ground system too. It can't.
The 50 ohm thing is part of the sales pitch- you don't need an "expensive tuning unit", just a simple series capacitor. And that may work fine in New Mexico where Nott is. :wink:
In ice country you need a T or at least an L network after finding a tap point where both R and X are reasonable numbers.
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kkiddkkidd
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Re: TUNING YOUR ANTENNA

Post by kkiddkkidd » Sat Oct 03, 2015 9:39 am

ChuckG wrote:Nott still suggests a skirt can compensate for a deteriorated or marginal ground system too. It can't.
The 50 ohm thing is part of the sales pitch- you don't need an "expensive tuning unit", just a simple series capacitor.
With todays no-knob transmitters, a unipole with only a series cap is a nightmare. I have several sites that have worked fine tuned to 50 ohms with a series cap but also have a few that required the installation of some manner of tuning to keep them stable. I have installed BE's tuning unit on a couple of Nautels now and have converted a few series caps to a real matching network just to get a handle on some minor instabilities.
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dd92251
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Re: TUNING YOUR ANTENNA

Post by dd92251 » Sat Oct 03, 2015 12:13 pm

There is no magic in the " 50 ohm" point of a Folded Unipole. In fact, as has been pointed out, the 50 ohm point may present quite a bit of reactance along with it. I agree with Deep Thought, find a point that minimizes the reactive part while keeping the resistive part above 20 ohms.

People forget that the reactive & resistive part together form the "impedance" Square Root of ( R squared + Z squared ) ....and that high impedance at the feedpoint can create some troublesome issues, like extremely high voltages at the feed bowl etc.

Been there and dealt with that...

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Re: TUNING YOUR ANTENNA

Post by ChuckG » Sat Oct 03, 2015 1:46 pm

kkiddkkidd wrote:.....
Aren't you supposed to be somewhere on the Riviera? :lol:
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Re: TUNING YOUR ANTENNA

Post by kkiddkkidd » Sat Oct 03, 2015 5:34 pm

ChuckG wrote:
kkiddkkidd wrote:.....
Aren't you supposed to be somewhere on the Riviera? :lol:
I wish I was there now... Actually leaving on the 15th. I have a couple FM projects to finish before we leave.
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