The T loaded will perform very similarly to the L loaded vertical in the
vertical component is about the same and the T and L are about the same
length -- one centered and the other offset. Generally, the objective of top
loading is to increase the feed point resistance with the idea of reducing
ground losses or making matching easier.
I have found that using a 55 foot radiator and a 45 foot L will produce
about 12 +/-j ohms at the feed point on 160 meters. That reading includes
resistance attributable to the ground resistance (maybe most of it is ground
resistance, I don't know) ( I am assuming that you have some sort of radial
system at the feed point ).
I have just finished modifying a HyTower to make it continuous from top to
bottom (connected stinger to tower at top of tower) and hung a 45 foot wire
from the top of the stinger toward another tall support (tree). My
measurements with a GenRad 1606A RF Bridge gave R readings on 160 on the
order of 12 ohms. The X readings were negative (as expected). I used a
series inductor to get X=0 ohms at about 1.825 MHz. Then I wound a 1:4 unun
(see Sevick's books) on a ferrite rod. The series inductor goes to the low
impedance input of the 1:4 unun and the output is about 55 ohms. The result
was a very flat VSWR curve across the low end of 160 meters. When I did the
matching with an LC network (or tuner) I had to make adjustments every 15
kHz or less to please the transmitter.
Tod, KØTO
-----Original Message-----
From: topband-bounces@contesting.com
[mailto:topband-bounces@contesting.com]On Behalf Of rmf01
Sent: Monday, September 22, 2003 11:01 AM
To: topband@contesting.com
Subject: Topband: T loaded vertical
Hello everyone,
I'm new to topband. Recently got on with a coaxial L at 55 feet and I'm
sure that it's no surprise to all that the performance is mediocre, although
I have worked 23 countries.
Thinking about erecting in it's place a T loaded vertical and wonder if
you have any suggestions on calculating the dimensions of the T if used with
a 55 foot or hopefully a 60 foot radiator. The T would have to be partially
strung through tree branches. Is a single wire radiator sufficient?
Finally, any suggestions on a matching? If it comes out at ~20 ohms can I
direct feed with the RG213 that is in place and use the tuner in the shack
or is it best to construct an L/C circuit at the base?
Tnx,
Rich W1TSP
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|