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Re: Topband: Air Wound Coil

To: W7RH <midnight18@cox.net>, Topband <topband@contesting.com>
Subject: Re: Topband: Air Wound Coil
From: Grant Saviers <grants2@pacbell.net>
Date: Sat, 1 Sep 2018 15:42:32 -0700
List-post: <mailto:topband@contesting.com>
Me too.

Or the inverse as I did, cut my T for the low end of the band. Then three series capacitors with PCB relays to short each individually (none, 1, 2, or 3) yielded nearly full band coverage <1.5:1 swr.  My T is 85' to top and a 50:25 ohm TLT is a close match, then the capacitor stack follows.  Caps and relays fit in a weather tight plastic box about 3x5x8".  Of course the resonance resistance and capacitor values depend on the antenna geometry. EZNEC easily models this antenna.  For me all three caps were 4000pf.  Relay settings of short, 4000pf, 2000pf, and 1333pf moved resonance in about equal 40KHz upward steps.  Instant QSY and cost about $100.
With 12.5 ohm resistance, QRO current will require 3 or 4 silver micas 
in parallel for each C.  Check the current ratings for the CDE CD/CDV16 
series micas.  Voltage is easily managed with 1000v rated caps since 
when in series they become a voltage divider.
Grant KZ1W

On 9/1/2018 13:17 PM, W7RH wrote:
Guys, I'm confounded by the complexity suggested for all band coverage of 160m "Top Loaded T Antenna".
May I make a simple suggestion. A simple T antenna will have a 
radiation resistance of 10-12 Ohms with electrical height of say 43ft 
with a Good ground system of quarter wave radials. By trimming the top 
load portion (both legs equal) to resonance on the high end of the 
band 12 turns of quarter inch copper tubing wound on a 4 inch ABS pipe 
with length of 12 inches and three relays would provide entire band 
coverage when connected to 12.5 Ohm Un-Un transformer. Cheap generic 
DPDT relays may be used to switch resonant points and can handle legal 
power limit.
Just sayin...
Bob W7RH

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