Craig,
OK, so you want to have an electrical quarterwave resonator and
electrical quarterwave radials.
I would suggest that you first cut two radials - make them a bit long
and put them up at the height and slope that they will eventually be
used. Connect these two radials as you would a dipole - then measure
the resonant frequency. Adjust the length as needed to resonate at your
desired frequency.
Note that these wires being close to the ground will not conform to the
usual "cutting formula" of 486/frequency in MHz, so start plenty long.
Trim the same amount from each end to keep both radials the same
length. You should now have 2 wires each an electrical quarterwave long
in the position they are to be mounted.
Once you have those two radials tuned - disconnect the one connected to
the coax center conductor (leave the one connected to the coax braid).
Connect the center conductor to the vertical radiator and adjust the
radiator length for resonance.
You will find this procedure in ON4UN's Low Band DXing book. He does
use a buried radial field to adjust the length of the radiator, but I
assume you do not have one of those handy.
OK, you have the vertical and one radial tuned for certain - remove that
radial and check that the other radial is also correctly tuned - adjust
its length as needed.
If you plan more than 2 radials, tune each of them (one at a time) to
resonate with the previously tuned vertical. When all have been tuned,
you may connect all the radials together.
If you want a simplified process that yields essentially the same
results (for practical purposes) - put up the vertical radiator cut to
the normal formula. It may not be exactly a quarter wave, but it will
be 'close enough' - yes it may produce something like a slightly
off-center fed system, but it will not be so far off center to make much
difference.
Now connecting each radial (again one at a time) trim the length of the
radial to make the resonance point at the frequency you desire.
Disconnect that first one and continue with all the other radials
planned. When finished with all of them, connect all the radials together.
That is my standard procedure for tuning elevated radials. With
elevated radials, only 2 are required to cancel the horizontally
polarized radiation - if they are placed 180 degrees apart.
73,
Don W3FPR
On 7/26/2014 9:27 AM, CRAIG W BEHRENS wrote:
I'm fooling around with custom portable Vertical array antennas for portable
and DXpedition use.
I was wondering if anyone in our list Brain trust had a favorite way to cut
wires to a (reasonably precise) 1/4-wave length.
(Yes, I can measure and No I don't want to model, I want to field test and
calibrate.)
What I want to do is have an elevated vertical (say base at 8'-10'), and add a temporary
1/4-wave "reference" counterpoise wire with a 20-degree downward slope.
Then, use this set-up to adjust whatever physical items I'm using for the
vertical element to get a 1:1 match, calibrated to the radials length.
I want to end-up with a balanced antenna that is resonant and does not require
a tuner (given that I'll have some minor adjustments to fine-tune the antenna's
match at different locations).
So, theoretically, the vertical could be made of 6-inch diameter aluminum
tubing (not likely in the field), insulated or non-insulated wire of any size,
or whatever.
I'm also curious about and I to be able to see what the differences are in the
lengths of antennas based on what materials of different types and sizes that I
use.
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