[TowerTalk] Clarification>. Bias Tees de K0FF

[K0FF]

Hi Tom, the "pair of paralleled .01's" are in the series arm. On the caps
specified, they are used in 8kW / 1.5-50 MHz rated lightning protectors as
the series arm. Same with the choke. Good point on possibly increasing the
value. Easy enuf to do that with a few more turns to give the desired
minimum rule of thumb > reactance = 10x impedance of feedline. I use 47's
and 100's (uH) myself, but that's just because I happen to have a large # of
them on hand.

Happy Homebrewing, Geo>KØFF
http://homepages.dstream.net/K0FF

Subject: Re: [TowerTalk] Antenna relay switching. Bias Tees de K0FF


> Hi George and Alan,
>
> >  Alan, if you want to make a high power "bias tee" for your application,
> >  you can use a pair of .01 at 3kV Z5U disc capacitors in series, with
>
> I'm not sure you mean to put the caps in series. That would be
> .005 uF. On 160 meters, it would be almost 18 ohms Xc, and give
> an SWR of 1.42:1
>
> If two .01's are in parallel, the Xc is about 9 ohms and SWR is 1.09.
>
> Voltage across two parallel .01uF capacitors with 1500 watts into
> 50 ohms is 50 volts. I'd stay away from really high voltage disc
> capacitors because they generally have more current handling
> problems.
>
> RF current would be 5.5 amperes, a multilayer capacitor would be
> better but some disks will work.
>
> >  about 40
> > uH
> >  of choke. One can be made with about 30 T of 26GA (or so)enamel wire on
a
>
> 40 uH might be a bit small for 1.8 MHz at 1500 watts, since it is
> only 432 ohms. It's right on the edge of being a problem.
>
> RF current through the choke would be .63 amperes RMS at 1500
> watts into a 1:1 SWR.
>
> Of course all of these rating assume a 1:1 load SWR. Things can
> get bad in a hurry as SWR goes up.
>
> It would be very wise, if you are unsure of the current rating of the
> capacitors, to use a whole bunch of them in parallel. I'd stay at
> least with 100 uH and make sure the choke has a fairly high Q.
>
>
>
> 73, Tom W8JI
> w8ji@contesting.com
>


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