Topband: Top loaded vertical help,

[Grant Saviers]

With 75' of vertical you have 56% of a 1/4wl which is about the net 
vertical dimension of my T with elevated radials.  My 2 top wires are 
about 36' long, ultimately they were cut to get a 25 ohm resonance 
around 1810KHz.  The antenna has about 70 KHz of 2:1 swr bandwidth.  I 
use a commercial 50:25 ohm UNUN and 240-31 multicore choke at the 
feedpoint.   Low loss, wide bandwidth, weatherproof, and very simple.    
Relay shorted fixed series capacitors on the output of the UNUN  tune 
the band in 50KHz steps.

Unless you are space constrained there is little benefit to 4 vs 2 wires 
for top loading.  The modeled azimuth gain asymmetry with 2 is about 0.1db.

As posted often, antennasbyn6lf.com and ON4UN's book have guidance for 
radial quantity vs length.

It's probably best to make 160 work before adding on 80m and then some 
modeling is likely important as it is with fan dipoles, which work with 
a single feedline.  For 80/40 fans I have 2.2' separation at the end of 
the shorter element.  For a 160/80 think more like 5'.  They will 
interact, so "cut and try" can be very frustrating. Check out ac6la.com 
in AutoEZ links for sample models on related topics.

I've never tried longer than 1/4wl verticals, but my conclusion from 
reading about them is that they aren't worth the trouble.

Grant KZ1W



On 9/5/2017 14:45 PM, MR TREVOR DUNNE wrote:
> Hi All
>
> Looking for a little help I'm going to build a top loaded vertical for 160m
>
> I have about 70-75ft vertical and was thinking about using 4 wires off the top to make up the difference,
>
> My garden is not in the clear with houses and other buildings surrounding it so I'm looking to get the max current up as high as possible,
>
> Would going longer on the antenna help maybe 3/8th wave will that effect the take off angle, also what would give the best match to 50ohms I'm not that clued up on matching networks so want to keep it simple as possible,
>
> I will be able to put a good amount of decent sized radials down 60ft minimum,
>
> I'm also thinking of hanging a 80m wire down along side the vertical spaced 2ft out and feeding it with another piece of coax against the same radial field will this cause me any issues ??
>
> I had a trap on a previous inverted L for 160/80 and the bandwidth was terrible so thinking this approach might work better,
>
> I'm a newbie to all this so be gentle with me Hi Hi
>
> Also what is a FCP seem it in a previous topic,
>
> Thanks
> Trevor
> EI2GLB
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