----- Original Message -----
From: "Jim Lux"
> I tried running two smaller (HF1250) amps each feeding one element of
a
> phased vertical. One amp receives a lot of reflected power from the
> other element. I would like to know if there is a circuit to stop
> reflected power but allow the forward power ?
>
Very interesting (since this is something I'm also working on doing)..
The short answer is that there isn't a "simple circuit" that will do
what
you want. In the microwave world, there are nifty things called
circulators
which do exactly what you want. However, a circulator for HF is
impractical
(haven't seen much waveguide around for 7 MHz, and I think that the
entire
world supply of ferrite wouldn't be enough).
You've got several practical (in varying degrees) approaches open to
you,
and you'll probably get lots of ideas (which I'd also like to hear!).
1) Reduce the mutual coupling of the antennas, either by moving them
farther
apart or making them physically smaller. You can still phase them, but
if
the mutual Z is smaller, you'll get less power coupled between them.
Negative power in an element implies a lot of mutual coupling (how close
are
the verticals?)
2) Create a LC network which connects both antennas to both amplifiers,
and
feeds the power appropriately. This might degenerate into a power
combiner
and phasing network
3) Are you really getting reflected "active" power or is it just
reactive
power circulating between amplifier and element? (that is, what's the
phase
difference between fwd and reflected signals).. In a driven phased
array,
there can be significant reactive power circulating in the feed line(s).
In
this case, a suitable matching network that supplies the reactive
power. If
you have one amplifier replaced by a dummy load, and you put power into
the
other antenna, how much power comes down the feed line into the load?
Jim, W6RMK
Reply from Steve
The antenna is two 80M 1/4 wave verticals 1/4 wave apart. The reflected
power is from mutual coupling and I see about 30 watts coming back on
the unused antenna into a dummy load with 100 watts into the active
antenna. I guess I can put a diode on the output of each amp and keep
the frequency clear on each side hi hi.
73, Steve, VE3TU
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|