I'll second Rick's approach, although I've not incorporated it yet. I built a
parallel wire transformer per Sevick Fig 6-11. It measures just fine. Until
it's installed I'm using an inverted-L with a shortened top wire which makes the
feed capacitive. A shunt inductor forms an L-network and is a built-in static
bleed.
Wes N7WS.
On 1/17/2022 11:34 AM, Tim Duffy wrote:
I match my 26 ohm feed point with a simple L network. One series coil then a
shunt capacitor. Steps up to 50 ohms perfectly.
73
Tim K3LR
-----Original Message-----
From: Topband [mailto:topband-bounces+k3lr=k3lr.com@contesting.com] On
Behalf Of Richard (Rick) Karlquist
Sent: Monday, January 17, 2022 1:10 PM
To: Kenny Silverman; TopBand List
Subject: Re: Topband: 1:2 UNUN
Maybe this will help:
I use a 1:2.25 Unun (3 to 2 turns ratio). It will match 22.2 ohms
to 50 ohms. It is wound on a single 2.4 inch diameter core of 77
material. A pair of RG-58 coax cables are wound around it in
tandem. I put on as many turns as would fit ... about 10 turns.
The shields of the two coax cables are wired in parallel.
This results in essentially a trifilar winding (1. The shields;
2. One of the center conductors 3. The other center conductor)
This trifilar winding can then be connected in the usual way to
get a 3 to 2 turns ratio autotransformer). The order of the
connections is: center conductor #1, then the shields, then
the #2 center conductor, in series. The radio drives
the 3 windings in series, and the antenna is tapped down to
utilize only 2 of them. I run this at 1,500 watts in
contests with no problems at all.
The concept is extensible to a quadfilar winding constructed
from 3 coax cables in tandem (shields in parallel). This can
be wired with a 4 to 3 turns ratio resulting in a 1:1.77 UNUN.
This matches 28.125 ohms to 50 ohms. An alternate modality
I have used is to use two coax cables as in the 1:2.25 UNUN,
but make one of the coax cables from tri-ax.
I can highly recommend UNUN's for the convenience factor if
your drive impedance is compatible with the available ratios.
They are broadband, no tuning.
73
Rick N6RK
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