Sinisa,
If you will agree to raise that value to 1000 Ohms, then I will agree with
you.
Often, 1000 Ohms is enough.
Steve (G3TXQ) has a nice chart showing impedances for various choke
technologies.
See: http://www.karinya.net/g3txq/chokes/
10x FB-31-1020 beads over RG213 will give you 1000 Ohms above 7.5 MHz, so I
assume if we use 11 beads, it will be 1000 Ohms at 7 MHz. Below that
frequency it drops to 500 Ohms.
I may be wrong, but I assume if you use two of these in series, you can
double the effectiveness.
So 22 beads would give us 2K @ 7 MHz., etc.
I have built and used the method I described in my previous post.
What I haven't tried but have thought about often, is simply wrapping RG-142
over a -43 toroid and using two of those piggy-back to feed with openwire.
17 turns will get you 8K Ohms down to 80m and about 4K on 160m. You can
just fit 17 or 18 turns onto an FT-240 core, but you have to wrap it very
tight.
The only thing is, I don't know if you would see any negative affect by
mounting these piggy-back with just a piece of Plexiglas (or similar)
between them.
Any thoughts on this?
73
Rick, DJ0IP
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of shristov
Sent: Monday, April 22, 2013 10:10 AM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Built in SWR meter bannans
Rick - DJ0IP / NJ0IP <Rick@DJ0IP.de> wrote:
> With openwire you can build a CMC choke just as easily as with coax.
> Only thing is, you have to build two of them so it costs twice as much.
> In fact you make your own coax CMC choke by placing beads over the
> coax and coating with heat-shrink tubing... you can just tape two of
> these together and use their center conductors for the connections.
In theory, yes, provided that the choke impedance is much higher than
antenna impedance.
In practice, this means choke impedance >=10 kOhm, which can be achieved
only with special attention, and then only within a narrow frequency band.
"Beads over coax" choke can hardly reach more than ~500 Ohms.
73,
Sinisa YT1NT, VE3EA
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