Topband: Traditional or off-center fed 160m vertical design?
d.cutter at ntlworld.com
Mon Nov 7 09:23:13 EST 2016
Have a look at this idea:
This is an off centre fed but your normal feeder is connected to the end of
the antenna via a choke. I've made several of the 40m version and 3 using
double the sizes for 80m. I used simpler chokes for both and measured
common mode currents to ensure it worked as an inverted L with no ground
radials. This is a variant of the sleeve dipole or "end fed dipole" often
used at VHF and above.
I and a friend are now making one for 160m but with a different choke and
balun suitable for the band. The Ruthroff will be about twice as long and
the choke will be on #31 core.
Give it some thought; it seems to me you have the facility (height) to make
a good one, whereas we are very much restricted in height.
My keenness for this idea is that radiation to the ground is minimised
because the real feedpoint is several metres in the air at the junction
with the Ruthroff. In my case I've arranged the transmitter attachment (end
connection) to be at ground level where the choke is grounded to minimise
common mode current back to the tx.
In my tests on the 80m version, I can reasonably estimate losses at about
5%, most of which is in the choke (a #43 with 16 turns of thin PTFE coax).
I used air tests with several precision thermistors and in pure water as a
calorimeter test running 100W. With suitable rating core and coax I don't
see why it shouldn't run at your legal limit.
----- Original Message -----
From: "Robert Fanfant" <rfanfant at hotmail.com>
To: <topband at contesting.com>
Sent: Sunday, November 06, 2016 7:16 PM
Subject: Topband: Traditional or off-center fed 160m vertical design?
> I am planning on putting up a 160m T vertical next week. After doing some
> modeling , I would like your thoughts concerning using a traditional ¼
> vertical design, versus an off center fed design.
> My modeling of the traditional designs approaches 36-38 ohms of real
> impedance while the off center fed design shows I can obtain close to 50
> ohms of real impedance.
> Trees on my property are roughly 143’ tall and I can’t use ground
> mounted radials for a variety of reasons. I found I can get up to 110’
> feet of usable vertical length, assuming radials @ 20’ off the ground.
> The antenna will be suspended between trees.
> I’ve discovered through modeling using elevated radials at 20’ , a 110
> vertical section. By varying both the radial and T top section lengths ,
> I can design a 160m vertical which approaches 50 ohms of real impedance ,
> using an off center fed design. It exhibits excellent characteristics from
> what the modeling shows. Based on the modeling I’m leaning towards the off
> center fed design primarily because It has a lower SWR at resonance
> (1.83Mhz) than the traditional vertical, and removes the need for
> building/adding a matching network if I only want to cover the lower
> portion of the band (CW). Thoughts?
> -rob N7QT
> Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows
> Topband Reflector Archives - http://www.contesting.com/_topband
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