I have always used the 468 formula for my desired frequency and put the
antenna up into the air with that length of wire. Due to various factors
this length is usually not correct, although it is "close".
I then calculate how much change 1-inch would make (that is .5" on each
side). Using this info I can then add or subtract wire to get to the
desired lowest SWR freq.
This works pretty well. My 2 dipoles (see below) have almost identical SWR
curves. It is close enough so that I can switch between them and not have
to retune.
I have a 40M/20M fan dipole up 75' in center and it is broadside roughly
NE/SW. The support "pole" is a real tall Pine tree. The ends are on other
Pine trees at about the same or slightly lower height.
I have a second 40M/20M fan dipole at about 74' in center (just below the
higher antenna) and it is broadside NW/SE. I was lucky enough to have
decent supports for this antenna but one end is only up about 50'.
These antennas have performed very well and haven't come down in any of the
storms that have moved through.
I just used #14 wire with black insulation from Home Depot.
-----------------------------------
Wes Attaway (N5WA)
Shreveport, LA
-----------------------------------
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
john@kk9a.com
Sent: Wednesday, September 21, 2022 8:56 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Inv v lengths
I concur. For me the 468 formula always needs trimming, perhaps because I
use THHN, so I do not initially cut it long. Once installed I measure the
resonant frequency and calculate the adjustment needed. easy-peasy
John KK9A
Gregg K9KL wrote:
Goodness. It's not that hard. My experience is 468/ freq. Cut it a foot
longer and put it up. You might have to fold some back on the insulators.
Just try it.
Gregg. K9KL
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