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Testing a coil for AM part 15

R

radio8z

Hello all,

Just wanted to tell you about an experiment and hopefully give some useful advice.

I constructed a coil:

Specs:

Coil form 3.5" OD PVC sch 40 pipe.

Turns 90

Measured inductance = 275 microhenries

Measure Q = 7.2

What is with this lousy Q? I used a Fluke 710-A and a HP 4250A impedance bridge and both gave the same results. Then I measured the DC resistance of the coil:
R = .215 ohms. The L and Q were measured at 1 KHz. I calculated the Q at 1 KHz. using Q = 2*pi*Frequency*inductance/resistance and got Q = 7.18. Super agreement with theory.

Why am I not happy? Because this in no way represents the Q at 1500 KHz. The resistance is the resistance at 1 KHz., not the resistance at 1500 KHz. (look up skin effect).

This is just for your fun, but I did note something else that has practical application. As I was taking measurements, initially, I used clip on jumper wires. I got Q values for this coil from .2 to 1.3 and they were not repeatable. It was only when I used #14 wire soldered to the coil and connected to the bridges that I got consistant and theoretically verifiable results.

So what? Well the so what is if you are trying different taps on your antenna coil to tune it to your transmitter, make sure you have a solid connection. Solder your test connections, don't rely on clip on jumpers.

Hope this helps.

Neil
 
> Hello all,
>
> Just wanted to tell you about an experiment and hopefully
> give some useful advice.
>
> I constructed a coil:
> REST DETETED
I was recently retuneing my AM antena and noticed the same
effect and proceeded to tack solder as I was selecting the best
tap and came up with different results than cliplead method.
I am using the alumiumn frame from a sliding door, a coil
I wound myself with RS 16GA speaker wire (striped so as single
conductor. Also using an old lid from a electric wock as the top hat.
It all looks rather commical but works well enongh for my
purposes; listening to "my kind of music" while walking my dog
around our development. It's located in my attic. I get about
a 1/2 mile without any special ground, only connecting to the
AC neutral conductor. May work on that as time allows and the
attic is not so hot. Oh yes I am using the SSTRAN XMITER.
Thanks for the interesting post

John Anderson
 
Hi John,

Nice to read your reply. I am glad that your are experimenting. Me too.
Build on the ideas and experiences of others and enjoy.

Neil
 
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