Rys asked about L networks, but perhaps others would like that information
too.
L networks at the base of the vertical are an excellent method for matching.
At the low end of 160m various programs or measurements show an impedance of
around 37 + j60 ohms. There are many articles on L network design, one from
W7LR in HR Feb. 1974, or in ON4UN computer programs, etc. Where the design
allows use of two capacitors then there is hardly any loss in the network. I
use
the
ceramic doorknob capacitors - see Surplus Sales catalog page 101. The
1000 pf capacitors are rated at 5 kv and 5.6 amps. For the low portion of
160 meters I use a 1000 pf shunt C at the end of the coax feedline, and series
C
of 2400 pf from that point to the antenna. For higher in the 160 meter band
I use 1000 pf shunt and 1300 pf series. With those two networks I can tx
anywhere in the band.
With the same antenna an L network also does the matching for 80 meters.
Here a series L from the coax to the antenna is used, and a shunt C across
the antenna. Sure there may be some loss in the coil but it is small compared
to
your tx power, or the big effects of changes in propagation, and it is a
better method than traps up on the antenna - better to have the antenna as
simple
as possible and any complexity on the ground. I use a rotary coil driven by a
screwdriver motor to tune between cw and ssb on 80, using low power and a
clear frequency to tune that for lowest swr. I could have used two fixed L
networks instead, as done on 160m.
My shunt C is 75 pf, and the coil is a Johnson variable inductor.
There has been a lot of discussion on radials. I am old fashioned and like
ground mounted radials. I use #14 housewire, even colored to be able to see it
better.
I just make them any length that fits the antenna field. I have 60 wires as
more is not worth it. Even 30 will work well. I don't worry about their
length vs 80 or 160 - just fill the field or lot where possible. The antenna
works well on both bands.
One other related topic is selection of the networks. I use regular 20 amp
plastic covered relays to switch on the 50 ohm side. On the antenna end I use
Collins
surplus open frame relays since the rf voltage is higher there. See Surplus
Sales catalog page 154, RNF-100 DPDT rf relay, 12v coil, 5 kv contacts.
Also use good guy rope material. I used to use nylon but dacron uv rope is
better - The Wireman is one source.
73 Bob W7LR
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|