[OKDXA] Dipole Antenna Question

[Nelson Derks]

> Okay I hafta ask.

You're a brave man...

> say our home brew ladder line hainging from the center insulater is
> pruneable, and it is. Would their be any way to tune the little bugger
remotly?

As a matter o'fact... YES.

In the case of the 40 Meter Extended Double Zepp presented earlier, there
would be little need to adjust the tuned feeder once it's dialed in. EDZ's
are very broadband and the antenna would likely cover all of 40 with an SWR
of better than 1.5:1 at the edges. It would also have a very wide sweet spot
of 1:1 SWR probably 250 kHz or so in width, and that's a big slice of the
band. In the case of the 20 Meter EDZ I had on the air for about a year, the
SWR curve was like 1.5:1 @ 14.000 MHz, 1:1 @ 14.060 MHz (that's the steep
curve at the low end) and it held a 1:1 SWR through 14.300 or so... At
14.350 MHz it was around 1.3:1. Fully plug & play across the band with no
tuner needed. The pattern was a tight Figure-8 and it was killer into
Hawaii.

Why is this so much better than a simple dipole?

The EDZ is nothing more than a three half wave Dipole with the center
section pulled down into a non-radiating tuned feeder. Since we're dealing
with three times the wire length, the percentage of change from pruning in
QSY'ing is also cut by a third. The antenna tunes more broadly. You can
think of the tuned feeder as a phasing line that times the half waves
presented to the 1.25 wavelength radiating section. This sharpens the
Figure-8 pattern for best gain perpendicular to the wire, and you can slide
the center insulator to shorten the top section to 1 wavelength tip-to-tip
and lengthen the feeder accordingly. Then it's called a Doublet and has a
slightly broader pattern. Take the ends of the Doublet and solder them
together, arrange the wire into a square, triangle, or circle, and that's
called a Full-Wave Loop. Are the gears between your ears starting to
turn...?

Now... Since we're tuning the total wire length and have discovered we can
arrange some sections to radiate while others don't, let's take this a step
further: Imagine you have some yard space that can comfortably fit 100' of
wire. No magic number here, it could be 85' or 112' just as easily.
Calculate an odd number of half wavelengths for 10 Meters (by using 492 / f
in MHz) that comes out to more than 100' of wire. My calculator says 7 half
waves come in at 120'. It also says 5 half waves are at 116' on 15 Meters.
We build a 100' antenna with an 8' tuned feeder and prune it to resonance on
15. Then we add an oblong box with relays inside. And lengths of ladder line
hanging below. A short piece of ladder line added should give us a match on
10, a longer piece would add 20 Meters. Calculate ladder line jumpers as
needed for the bands you like and you can tune your 100' antenna anywhere
you want by switching a series of relays. For the truly mechanically
inclined, there may be a spooling option with sliding contacts on bare wire,
or you could use a model train engine and call it The Lionel Antenna... It's
the concept that counts.

(Hey, I think I just found the next advert for ShaZamaHamaTronics! Combine
two popular geezer hobbies and you know someone will buy it!)

The outdoor relay idea looked like a maintenance problem to me, and the
pattern will vary from band to band, so I ended up with an Off Center Fed
Dipole (Coaxial Windom) that uses ~137' of wire fed ~46' from one end
through a 4:1 current Balun and tunes everything except 160, 30 and 15
Meters. 75 Meter 'fone needs a little help from the antenna tuner in the
rig, but otherwise it's plug & play all the way from 80 CW up to 6 Meters. I
could add a second pair of wires for 30 and 15 Meters, but I'm having enough
fun as is...

Now, aren't you glad you asked?

And... Yes... You could take a pair of 102" CB whips and feed them with a
very long ladder line tuned until it resonated on 75 Meters. The efficiency
would be poor, but you'd probably make it out to Union City on a good night,
and that's all that counts... Or, if your call is KD5DLL, you can forget the
feeder and just use the CB whip on 27.185... Hellllooooooooooooooo
Newfoundland!

- AC5UP