Vytenis,
Sounds like a great antenna!
The formula for changing it is a simple one ... first measure the actual
length as is. Then put it up and measure the point where the reactance, X,
equals 0. That is the resonance point. Now use the ratio of that frequency
to the one you want to have set. That's it.
if the desired frequency is higher than the actual than make the antenna
shorter by that percent ...
new height = current height ( current frequency / desired frequency )
Assume current height = 21.0 M ... measure the X = 0 point and it is 3450.
Desired frequency = 3550.
new height = 21.0 ( 3450 / 3550 ) = 20.41M
if the desired frequency is lower than the actual than make the antenna
longer by that percent.
new height = current height ( current frequency / desired frequency ) )
Assume current height = 21.0 M ... measure the X = 0 point and it is 3650.
Desired frequency = 3550.
new height = 21.0 ( 3650 / 3550 ) = 21.6M
using the lowest SWR point will muddy the water a little bit but it too will
get you in the ball park.
Gary
K9RX
-----Original Message-----
From: V. Sciucka
Sent: Wednesday, September 13, 2017 9:51 AM
To: towertalk@contesting.com
Subject: [TowerTalk] 80m vertical size calculation
Hi guys
We at club station going to build 80m full size vertical. We have assembled
it on the ground and it is 22m (72,2ft) length. What we afraid is that
lowering and raising again procedure will be quite tricky. Therefore we
want to get into resonance as quick as possible. Reading all available
literature and web resources gives very different values in length: from
58ft to 68ft. What is recommended length to get to lower part (~3550) of
band? Material is aluminum tubes with bottom diameter ~4in and top tube
under 1in.
Second question, if I find antenna resonates lower than I need, lets say
3440 is there a practice or formula to calculate how long tip should I cut
off? Or just "cut a feet or two, raise again and see difference"
In the future, hopefully quite soon, this will be part of classic 4SQ.
--------------------
Vytenis LY5T
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|