Topband: Air wound coil

[Richard Schmiedt]

Jim,

I have that 80m quarter wave vertical by DXE and use a simple matching network with it for 160m.  As it stands, the vertical is basically an 1/8 monopole with ~12 ohms real Z on 160, so I use a 4:1 balun to bring the 50 ohm coax down to 12 ohms then add a 4 inch diameter coil in series to resonate the stick to 1850 kHz (as I remember about 23-28 uH).  It works very well with 28 33’ radials and 10 65’ radials underneath. The 2:1 bandwidth is around 40 kHz.  (The QTH is near Charleston SC near a marsh.)

I feed the coax into a T connector at the vertical, one side going directly thru a Normally Closed, 12V vacuum relay to the feed point of the vertical.  This bypasses the tuning circuit and resonates the vertical on 80m. When 12V is applied to open the relay, the RF then goes thru the balun and coil which is hardwired to the feed point of the vertical (at the same point as the output of the relay).

It’s simple and it works. The 4:1 balun by Balun Designs is < $100 and the coil I bought at a flea market for $30. It’s flat wound so it’s easy to solder taps onto it.  I use flat copper strips to solder to the coil. The vacuum relay is rated at 1500V 4A QSK found on eBay for ~$35. 

I haven’t tried switching taps to gain BW. I use a tuner at the rig and it will go from about 1800-1925 at 1200W without much of a problem. Above 1925, the Balun really starts to overheat so the power has to come down a bit. 

Rick
W4GE

Sent from my iPhone

> On Sep 2, 2018, at 6:03 PM, Jim Garland <4cx250b at miamioh.edu> wrote:
> 
> On this general topic, I'm erecting a 68 ft aluminum self-supporting pole (DX Engineering) that is four inches OD at the bottom, tapering to about 1/2 inch at the top. It is designed as a full size 1/4 wave 80m vertical. I'd like to use it on 160m. If I add four capacity hat wires at the top, would that sufficiently lower the resonant point? The antenna tilts at the base with a hand winch, so it's not hard to add or remove wires if I want to restore the antenna to 80m.  The other option is to add inductance at the base (easy to do), but I don't know if that is the preferred solution or not. BTW, there are sixty 30m radials already installed at the base, from a previous antenna. I only care about resonating the antenna near the low end of the band. Except for cacti and coyotes, the antenna is 200m from any structure, fed with buried hardline. Any suggestions? Thanks!
> 
> 73,
> 
> Jim W8ZR
> 
> 
> 
> _________________
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