Hi Terry:
I didn't go much beyond the simple LC matching that just jumped out at me
when I saw the plot from the attached EZNEC file. I can't absolutely say
that I can get this pattern from a real world build, but it looks
encouraging.
I've done remote relay switching before, in fact my 80/40 vertical does just
that using two vacuum relays. What's interesting about a remote matching
system, other than the need to supply switchable DC at the antenna, is that
with the addition of a 150 pF capacitor to ground just ahead of the 0.25 uH
inductor you can match the antenna to very close to 50 for the feedline. It
requires a bit of messing around with the length of the two dipoles but it
can be done, at least as far as EZNEC is concerned. I've got an HP8753C VNA
so when I get around to putting the wires up I can at least see what the
feed point impedance of each dipole is and then design any matching network
around the measured values.
Of course all of this will have to wait for cooler weather and the lack of
Hurricanes here in Florida. Luckily I'm on the west coast so it shouldn't
be too bad. Went through three of them in 2004, with barely a small amount
of broken tree limbs in the yard.
Cheers for now,
Karin
-----Original Message-----
From: Terry Conboy [mailto:terry.conboy@gmail.com] On Behalf Of Terry Conboy
Sent: Thursday, August 29, 2019 12:56 PM
To: Karin Johnson
Cc: antennaware@contesting.com; weinfurt@ohio.edu
Subject: Re: HF Colinear Dipoles
Karin,
It's hard to beat horizontal antennas on 20m and up due to the ground
reflection gain. Same for 40m if you can get a decent height (> .5 wl).
As for matching the switched 2-element collinear, after I sent out those
models, it occurred to me that you feed them quite simply with 3/4 wl 75 ohm
lines from each antenna, then in the shack, use two 1/4 wl 75 ohm lines
connected in line with the two feeders. For in-phase, feed at the junction
of the two 1/4 wl lines (total 1 wl to each antenna) or for out-of-phase,
feed at the junction of one 3/4 wl feeder and a 1/4 wl line (so 3/4 wl to
one element and 5/4 wl total to the other). Due to the mutual impedances,
the elements are ~100 ohms in-phase and ~50 ohms out-of-phase, so this
should give you 50 ohms at the common point for both patterns and switchable
0 - 180 phase with just a SPDT switch or relay.
There shouldn't be a need for L & C matching, unless you are really picky
about SWR. Of course, if you use wider spacing between the elements, the
element drive impedances will move closer to 75 ohms and things get more
complex.
A bigger question that has always bugged me: why are there two L's in
collinear?
73, Terry N6RY
> On 2019 Aug 28, at 9:50 AM, Karin Johnson <karinann@tampabay.rr.com>
wrote:
>
> Hello Terry and Greg:
> I appreciate the responses and the models. I also received an email from
> Gedas who pointed me in the right direction. First off let me say I'm not
> a novice to this task. Although most of my efforts on antenna design in
> the professional space have been in the 2 GHz and up arena. I've actually
> got some patents on some of the designs. With regards to that task HFSS,
a
> very expensive modeling software is a joy to use. Now with respect to HF
> antennas. What got in the way for me was the matching task and the
> influence of one dipole on the other.
> Although the Low Band DXing book addresses verticals the basic concept of
> mutual coupling still exists. You can see this effect if you model two
> collinear dipoles, look at the source data, then remove one of the dipoles
> and look at the source data. The source impedance at the generator will
be
> different. Gedas sent me an article he wrote some time ago and this
> set off the light bulb in my brain. From there I've been able to make
some
> progress. The main reason for me doing all of this is curiosity. Right
now
> I have a 40 meter dipole in place and have recently put up a switched
80/40
> meter vertical. I find that I am almost always using the vertical now and
> have toyed with the idea of a gain antenna for 20 meters to take the place
> of the 40 meter dipole. I also have another 20 meter dipole that will
> remain in place but will be oriented 90 degrees to the collinear array if
I
> decide to build it. So for now this is mainly an academic exercise. With
> regard to some of the simulations I have done with EZNEC I've found a
really
> simple way to match both the in phase and out of phase arrangement of the
> two dipoles in collinear orientation. It does require some vacuum relays
at
> the junction point of the two feed lines but this is very doable. I also
> need a 150 pF vacuum cap, and a small 0.25uH inductor. I know vacuum cap
> probably is overkill but I tend to over design things.
>
> Regards,
> Karin K3UU
>
_______________________________________________
Antennaware mailing list
Antennaware@contesting.com
http://lists.contesting.com/mailman/listinfo/antennaware
|