Begging**K5PGW's permission I continue:
There is no need for the radiator to be longer than an electrical 1/4 wavelength
in order to shunt feed it, in fact the converse is normally preferred. It's
easy to consider the input to the Gamma as the feedpoint and want it to be
inductive so that a series capacitor can resonate it, but the actual feedpoint Z
for calculation purposes is the base of the vertical when detached from ground
and series fed.
There are programs out there to aid in designing these systems, with varying
usefulness, and they begin with the base impedance. An interesting paper that
covers a lot of this is found here:
https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-magloop-gamma-6MW.pdf
Another paper I mentioned earlier also talks about slant wire feeding 60 degree
high towers.
There are a number of pitfalls when using EZNEC to model closely spaced wires,
which is what a gamma match requires, however, with care it can be done.
Segments must be equal lengths and aligned and not too close together and
connecting wires must connect at segment junctions and be similar diameters.
AutoEZ makes this easier because of its fantastic optimizer and is a good way to
explore these designs.
Wes N7WS
On 9/5/2019 6:00 PM, VE6WZ_Steve wrote:
Scott…
I am guessing you have some Yagis on top of that tower.
Like you there are many towers shorter than 1/4 WL long (including mine!) that
are shunt-fed.
Its the electrical length that matters. Top loading works.
It seems you didn't watch my video, since I actually show two images from
ON4UNs book referring to calculating electrical length.
So…yeh…I’ve seen his book.
On Sep 5, 2019, at 6:27 PM, K9MA<k9ma@sdellington.us> wrote:
I must disagree. I’ve been shunt feeding a 21 meter tower on 160 for years,
with a simple gamma match. At most, it is electrically 3/16 wavelength.
See ON4UN’s book.
73,
Scott K9MA
----------
Scott Ellington. K9MA
--- via iPhone
On Sep 5, 2019, at 5:40 PM, VE6WZ_Steve<ve6wz@shaw.ca> wrote:
Your tower is too short for a shunt-feed. It needs to be inductive at the QRG
of interest, 90 degrees or more (1/4 WL).
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|