[Lowfer] 600MRG> Why put a coil under the top hat ???

[EVP]

  The L/D of the coil isn't very critical at all. Actually turns out the 
optimum L/D for maximum inductance with a given length of wire is about 
1.6 but the loss is hardly different for ratios quite different from 
that. You are correct that the purpose of moving the coil up the antenna 
is to increase the current near the top. Unfortunately as you move the 
coil up the inductance has to go up because the capacitance comes down 
and with the higher inductance will go higher losses unless you make a 
gigantic coil. Even if you do the bandwidth will get narrower. All of 
this applies to a good ground circuit. With a poor ground the losses may 
not make that much difference and a higher coil may indeed help. About 
20 years ago one of the local Socal lowfers [forget who it was but he is 
no longer with us] actually put a variometer directly at the top of the 
vertical wire and tuned it with a nylon line around a pully on the 
shaft. It worked but he ended up with not much measurable increase in 
signal at my location, which is about 30 miles away.

Ed

Andy - KU4XR wrote:

>Hi All:
>
>I hope everyone is having a peaceful Christmas, and HNY 2010 to you.
>
>Since I posted the original question on this topic, I thought a
>follow up might be in order. The information I wanted is primarily
>to see if I can improve my 1750 meter antenna any further, so PSE
>forgive the intrusion into the 600 meter group. I have been reading
>lots of material on coil placement, and dimensions the last few days
>Cebnik, Rauch, Flory, and Koehler. All of them pretty much say the
>same thing; Center placement will provide optimum results for a
>very short antenna with limited top hat length.
>I am still not persuaded on the optimum L/D of the coil though.
>I'll use thin walled 4" white PVC as a form, wound with #14 AWG
>with thin insulation ( 15 amp rating ). A 2:1 ratio allows for
>about 68 turns over 8" for around 189 Micro-Henries.
>A 4:1 ratio allows for about 136 turns over 16" and about 415 uH.
>At 185 KHz; neither of these coils provide a significant load
>in the antenna line I don't think. It would appear that the biggest 
>advantage would be in getting the current farther up the antenna,
>and the hot end of the coil about 15' above ground. I will need to
>rewind my variometer at the transmitter for fine tuning.
>Finally; IF !! adding a center mounted loading coil will make the 
>antenna more efficient than base loaded alone, I certainly want 
>to give it a try. The only question I have now is on the optimum
>L/D ratio of the loading coil. Any replies on this matter will
>be appreciated, and thought thru.
>
>73 all :::
>
>
>
>Andy - KU4XR - EM75xr - Friendsville, TN.
>LOWfer Beacon " XR " @ 185.29866 KHz ( QRSS-60 )
>Coordinates:  N:  35* 43' 54" - W:  84* 3' 16"
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>http://webpages.charter.net/ku4xr/
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