To clarify my earlier posting:
I am only talking about replacing the amplifier at
the bottom of the whip with a transformer. You still
of course need a phasing network. This phasing network
will be the same whether you use the DXE amplifiers or
you use transformers. You can use the DXE phasing
network if you want. As was pointed out before, receiving
phasing networks are relatively easy to build because
you don't have to worry about mutual coupling, especially
with 13 ft whips.
The low impedance side of the transformer is connected
between the center conductor and shield of the coax.
The high impedance side is connected between ground
and the whip. There is no ground connection to the
coax. This is exactly like a Beverage transformer
hookup.
There are several thousands of ohms of capactive reactance,
depending on the frequency. The transformer does not tune
this out or or in any way provide a conjugate match. The
optimum transformer ratio occurs when the resistive load
presented to the antenna equals the magnitude of the capacitive
reactance. However, you can be off by a factor of 3 or 4
either way with only a few dB of lost sensitivity. The loss
of sensitivity is not a real loss in what you can receive,
it just means you will need a few more dB of gain in your
preamp. The preamp is a plain 50 ohm to 50 ohm or 75 ohm
to 75 ohm amplifier. It can be preceded by an ordinary BCB
rejection filter if necessary. You cannot precede the DXE
amplifier with a filter. Another reason not to use it.
Rick N6RK
Jim Lux wrote:
> At 12:18 PM 12/18/2006, Rick Karlquist wrote:
>>Stone, Gary R. wrote:
>> > Greetings,
>> >
>> > ALSO - how about the unit being sold by DX Engineering - don't know
>> the
>> > model number but it uses (4) 102 inch whips. It seems very pricey for
>> > the system but I am curious. Any homebrew plans around for using (4)
>> > 102 inch whips? (they are easy to find and cheap).
>> >
>> > Gary, N5PHT
>>
>>Save your money. You can do just as well by using a passive
>>transformer to connect the whip to your transmission line.
>>I actually tried this in an A/B test with the DX Engineering
>>amplifier. You might start with a Mini-Circuits T16-6 which
>>will load the antenna with 800 ohms (or if you use RG-6U, 1200
>>ohms). That will be within a few dB of optimum sensitivity.
>>Try modeling it on EZNEC and you will see what I mean.
>
> Do you do anything special to deal with the huge capacitive
> reactance? Or, just assume that the big resistive load swamps it?
>
>
>>Rick N6RK
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