TopBand: Three eighths wave inv-L

Tue, 29 Jul 1997 09:38:03 +0000

> To:            <topband@contesting.com>
> Date:          Tue, 29 Jul 97 12:00:14 +0000

Hi Steve,

> The antenna is sited remotely from the house, in order to cut down
> electrical pickup from domestic appliances, so the simpler the network is to
> tune/configure the better.  It is fed against an elevated G6XN-type
> counterpoise.

That's a pretty poor ground to use if you have room for radials. 
Everyone in VK has a ten thousand acre ranch, don't they? ;-)

> At present the antenna has a 140' horizontal top, with a 60' vertical
> section, and is fed with 50 ohm coaxial via a series capacitor of  around
> 350pf. Although I can tune out the inductive reactance with the series C,
> the antenna is showing an SWR of around 5:1 at the antenna (3:1 in the shack).

That's strange. Assuming an even half way proper meter, SWR should 
be consistent along a transmission line, except for a reduction with 
large distances if the feedline has high loss.  If the SWR measures 
different at both ends you most likely have either very lossy cable 
or severe common mode current on the feeder.

I suspect the poor ground system has given you common mode current 
problems, with the antenna trying to ground itself back through 
the OUTSIDE of the coax to your shack and equipment. When you move 
the meter you disturb that current, and change the system's resonant 
frequency or SWR.

> I can bring down the feed impedance towards 50 ohms by shortening the
> antenna by 30' or so, but would like to keep the dimensions roughly as they
> are, in order to keep the present current distribution/radiation pattern.

The 1/4 wl flattop L has considerable current in the horizontal flat 
top. Unless you want to work very short distances via skywave, most 
of that current is wasted, producing high angle radiation. A 
better goal, for contacts further than a few hundred miles, is to get 
maximum current in the very middle or even lower area of the vertical 
radiator. That maximizes vertical radiation, rather than wasting 
radiation generating a short distance high angle skywave signal.

Because of all that, I'd just chop off the thirty feet.

> The obvious way to match the antenna seems to me to use an L-network, which
> is switched by relays for 80/160m and uses a servo-tuned capacitor, but
> perhaps there is a simpler solution?

That should work. An L net is the simplest other than a simple single 
component.

> I would be especially interested to hear from anyone who has changed to this
> kind of antenna from a dipole.

I did that early on in my 160 activity, about 1964 Steve. In my case 
I was trying to work DX, and we were only allowed 25 watts in some 
parts of the band. I eventually settled on a top loaded vertical with 
a good ground system and three wires (one every 120 degrees) for top 
loading . 

I found the top loaded vertical, with a good ground system and max 
current near the base, to be a much better groundwave and DX antenna 
than the long flat-top L, although I couldn't tell much difference 
between it and a short flattop L.

73, Tom W8JI 

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