What test gear do you have available?
How much cut and try are you willing to do, and do you have some convenient
beacons/sources to test with to measure F/B (for instance)
There's sort of two general approaches to network synthesis for phased
arrays:
1) Figure out what element currents will produce the desired patterns (by
using NEC and trusting it), then, measuring impedances and designing the
appropriate network to feed it. (I've got a spreadsheet for Christman type
feeds that lets you figure it out, as long as you have the mutual Z matrix
for 2,3, or 4 elements). Measuring the Z matrix (which is essential for
this kind of approach) is somewhat tedious, but reasonably doable.
2) Use numerical models to get close, then do field testing to make the fine
adjustment. Taking the Christman feed for example, you'd get a bunch of
short jumpers of various lengths and systematically try different feedline
lengths until the performance is what you want. At 40 or 80 meters, you
could probably use hunks of coax without connectors and just twist the wires
or use crimpon lugs for testing purposes.
3) You can use an approach described in one of the Antenna Compendia and use
a standard antenna tuner as a phasing network and adjust it for optimum
performance. You could then measure the tuner and build a more rugged,
higher power version for permanent use. Something as simple as a MFJ CLC
type tuner could be used effectively, albeit tediously. With this approach,
suitable only for 2 element arrays, you use a DPDT switch to actually flip
the phase back and forth on one element to alternately null and peak the
desired signal.
If you have the ability to measure element currents (i.e. you can build some
current probes, etc.) then you've got that much more information to help you
tune.
Here's the tricky thing about measuring and cutting.. a lot depends on your
antenna model, because you're not actually measuring or controlling the far
field pattern, and it's unlikely you have a nice idealized situation with
your two verticals sitting out on a grassy salt marsh 100s of meters from
the nearest conductor. Other "stuff" near the antenna element will change
it's impedance (why you actually need to measure the Z, and not just trust
NEC), and, for the same reason as it changes the Z, will change the
element's far field pattern, which changes the array's far field pattern.
I've got a bunch of loosely organized stuff at
http://home.earthlink.net/~w6rmk/antenna/phased/index.htm that might be of
use, including details on current probe design, Christman design
spreadsheets, etc. If a link is broken, or you can't find something, let me
know, because it probably just means I screwed up moving the site around, or
I forgot to post it.
----- Original Message -----
From: "Col" <col@v21mail.co.uk>
To: <towertalk@contesting.com>
Sent: Wednesday, September 10, 2003 3:11 AM
Subject: [TowerTalk] 1/8 wave spaced 80m verticals
> Can anyone help me with the construction of a lewallen phasing network for
2
> 1/8 wave spaced 80m verticals or sort out my christman feed. I have used 2
x
> mfj1792 top loaded verticals. I have 40 radials 1/8 wave long on each
> antenna. I have tried the christman feed to these antennas but i suspect
the
> phasing is not quite right as front to back is only about 15db. I used the
> design in on4un low band dxing, 29 degrees and 157 degrees. I have
> subsequently learned that this will be specific line lengths for full
> size1/4 wave verticals.
>
> I have limited test gear available so I thought the christman feed would
be
> the simplest to construct. I would like to improve the front to back on
the
> array, any thoughts
>
> many thanks Colin GM0RLZ
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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