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G0FTD > COILS 17.05.09 14:01l 99 Lines 3641 Bytes #999 (0) @ WW
BID : 000631136PZT
Read: GUEST
Subj: Coils and things
Path: IZ3LSV<IK2XDE<DB0RES<ON0AR<GB7YKS<GB7SYP<GB7PZT
Sent: 090517/1031Z @:GB7PZT.#24.GBR.EU [Kidderminster] #:63100 XSERV410h
Date: Sun, 17 May 2009 11:31:00 +0000
From: g0ftd@gb7pzt.#24.gbr.eu (Andy)
To: coils@ww
Subject: Coils and things
Thanks for the interesting info. I thought a generic reply would
be of use to other readers (must be all of about 4 left in Packetland
by now).
G4EBT wrote:-
>There's been a lot of discussion in Technical Topics over the year on
>losses in 80M and 160M ATUs.
Eh ? I thought TT was dead, how old is this bulletin ? ;-)
Thankfully I have the complete collection on book and cd.
Well worth the money and for once I can thank the Potters Bar Book
Company for publishing it - as I'm sure David will agree.
>On 1.8 and 3.5MHz the ATU is often required to match the transmitter into
>antennas with a very low or very high feed impedance. Under worst case
>conditions, where the step down impedance transformation is the greatest,
>power losses - even on 3.5MHz, can climb to 6dB or more. This means that
>as much as three-quarters of the power delivered by the transmitter (even
>more on 1.8MHz) may be dissipated in the coil.
Too true. I have experimented with different coil styles, here's a
few results for tuning my 8m trolley vertical with two 15m long radials
at a freq of 1.9Mhz.
*A 70uh coil, about 1 inches long and 1.5 inches in diameter works
well and remains cold. However it did not have the ability to be varied
except at fixed points which was not good enough.
* a 70uh coil was wound on an old 35mm film cannister with two layers.
It worked but very hot. Clearly lossy.
* A re-wound variometer on bakelite worked superbly for matching
purposes and variability. However the method of connecting the inner to
outer coil just wasn't up to the job, the arcing started and melting
occured ;-)
* an inductor was wound on 3 FT140-61 stucks together got hot too.
I now have a 30uh rollercoaster which I will attempt to use with
a fixed inductance wound upon something suitable like ceramic
if I can find one.
As an aside, the voltages on my vertical on 160m are quite frightening.
A few years ago my thumb came close to the antenna whilst transmitting.
It zapped across through my thumb nail, burnt a hole in my thumb nail
and drew blood. It stung for about 2 hours too ;-)
>The critical factors are the maximum value of the input and output
>capacitances and the Q of the coil.
Which dictate the sizes and working voltages. Fine of you drive a tank,
but trolly pace can be a limiting factor, hence my experiments to
find the most compact solution.
Inductors are quite strange beasts.
When used in ATU's there can be problems with self resonant freqs
or Q problems.
Measuring the dratted things can lead to some very erronious values
too. The measurment device and frequency play a huge part.
Not long ago I rejected some TOKO coils from a large international
supplier. I had ordered 47mh but upon measurement with an LC
bridge at 30Mhz showed exactly 4.7uH. I sent them back claiming
a that there must have been a labelling or storage bin error
and requested replacements.
The same happened again and again so I started to question my
measurements.
My preferred method now is to measure the coil with a series
capacitor. I then vary the frequency of whatever impedance bridge
I'm using at the time and look for a dip close to zero ohms at
the frequency that I have calculated to work with the series C,
assuming the inductors value is correct.
It is also interesting to see how the Q is affected too.
As the coil is fixed with a variable C, a swept freq measurement
shows how the width and depth of notch (for example a series LC
circuit) can vary a great deal.
- &y -
G0FTD @ GB7PZT
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