[TowerTalk] Shunt Feeding a Vertical an easier wat
Joe Subich, W4TV
lists at subich.com
Fri Sep 6 15:58:59 EDT 2019
On 2019-09-06 3:47 PM, David Gilbert wrote:
>
> If you make the combination of the length of the radial(s) plus the
> length of the tower above the radial(s) equal to an electrical half
> wavelength, everything on the tower below the radial(s) is choked
> off. You can easily see this by looking at the current distribution
> in EZNEC.
EZNEC misses the effects of connection to real (lossy) ground. NEC 4
show significant issues when feed/control line and lossy ground are
(correctly) included in the models. The current below the feedpoint
is no longer zero or very low.
73,
... Joe, W4TV
On 2019-09-06 3:47 PM, David Gilbert wrote:
>
> If you make the combination of the length of the radial(s) plus the
> length of the tower above the radial(s) equal to an electrical half
> wavelength, everything on the tower below the radial(s) is choked off.
> You can easily see this by looking at the current distribution in EZNEC.
>
> So yes, N4KG's approach should work fine, although I don't see how it's
> any simpler than using a shunt feed. Both require you to find a
> suitable point on the tower, and I'm not sure trimming radials is any
> easier than adjusting a capacitor.
>
> It seems to me that the advantage of N4KG's approach would be that wire
> is cheaper than a good quality capacitor, and possibly that elevated
> radials require less wire than a radial field.
>
> 73,
> Dave AB7E
>
>
>
> On 9/6/2019 9:09 AM, Dave Thompson wrote:
>> Gang,
>>
>> This is my annual request that there is a simpler way to feed a
>> tower. Tom N4KG wrote an excellent article on his reverse feed system
>> in the April 1994 QST. With Tom's system using elevated radials
>> (either loaded or full length) you feed the coax to a radial and the
>> shield to the tower. We used a MFJ 259 to find a place to attach the
>> radials for a match on the tower. Tom also had an excellent section
>> of calculating top load to get electrical length. If you don't have
>> the actual QST you can down load the article.
>> Cebik contributed an article on reverse feed of towers and showed that
>> there little ground loss that W8JI and others predicted.
>>
>> We did two towers one 60 feet and one 90 feet and the results were
>> very good with great DX. Tom ran several towers and most low band
>> DXers know his results. Sadly Tom became a SK last year. Best DX was
>> Zone 17 on CW and Zone 25 UA0 from GA on SSB.
>>
>> 73 Dave K4JRB
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