[TowerTalk] Question on Multiple Inverted L Antennas

[hasan schiers]

I have an 80m inverted L up and running very nicely with 26 x 60' radials. 
I'm thinking about adding 40m. My support for the 80m L is at 46' on a 
pulley connected to a cross-boom 5' out from my tower. It works VERY, VERY 
well.  I'm going to put in another 24 radials, for a total of 50. Seems a 
bit wasteful not to get double duty if I can.

I am happy to use a tuner in the to match the variations in VSWR (as long as 
they are reasonable). Right now my feedpoint Z is 29 ohms (at the feedpoint) 
on my 80m inverted L (at resonance). I feed it directly (no matching at the 
base) via my LDG AT-1000 autotuner.

The simplest approach would be the two-wire, one feedline..although I 
haven't figured out how to do the 33' wire for 40m in terms of keeping it 
separated from my 80m L without having the wind blow it and wrap it around 
the 80m L.

Another method involves feeding the 80m on 40m as a half-wave vertical, and 
simply switching in a parallel resonant circuit (identical to the base feed 
of a Bobtail Curtain or Half-Square). This, of course, involves a remote RF 
switch and then making the matching network. In this case the high-Z feed is 
desirable, and not a disadvantage at all. (I've done this before for a 40m 
half-square and got the article published in ham radio magazine)

The simplest is still the single added wire...if I can figure out the 
mechanical problem of how to "spread" and "attach" it to the 80m wire so it 
won't "foul".

Ideas anyone?

73,

...hasan, N0AN
----- Original Message ----- 
From: "K4SAV" <RadioIR at charter.net>
To: <towertalk at contesting.com>
Sent: Thursday, July 13, 2006 1:11 PM
Subject: Re: [TowerTalk] Question on Multiple Inverted L Antennas


> The biggest problem you have with low band multiple L antennas is
> matching and bandwidth. To start with, a 160 meter L will not have a lot
> of bandwidth and the impedance will be low compared to 50 ohms (assuming
> you have a good ground). With a poor ground you can have a good match
> but a lossy antenna. Of course you have the popular option of making the
> L long then adding a series capacitor at the base and using that to tune
> the antennas to the spot you want.  An 80 meter L will not cover both
> the CW and phone bands, but you can do the same as for the 160, make it
> long and add a series cap to tune it to different frequencies.
>
> OPTION 1: If you feed three L's from a common line, they will interact
> in much the same way as a fan dipole does. You will have to
> experimentally change the length of each until you get the lowest SWR on
> each. The 40 meter portion will require a much longer wire than you
> expect.  After you finish, the SWR won't be very good on any of them.
> Also the bandwidth for each of them will be reduced from that of
> separate antennas (which are already too narrow). The 40 meter L will
> operate more like a horizontal antenna than a vertical (which isn't
> necessarily bad). This is a real PITA option.
>
> OPTION 2: If you separate the antennas (3 ft or more) and feed them
> separately, the tuning will show little interaction. Bandwidth will
> still be narrow as it is with separate low band antennas. With a good
> ground, you will probably get 100 to 150 kHz bandwidth on 80 meters. The
> 40 meter vertical will look like a vertical antenna. The 40 meter will
> have its feedpoint impedance reduced somewhat from that of a separate
> antenna, maybe down to about 25 ohms with a good ground. You may want to
> improve the SWR on these antennas. One way would be to make the antennas
> long, then use a single series capacitor to tune out the reactance as
> mentioned above. This would be a simple matching network, and you could
> have two caps for the 80 meter L, one for 80 and one for 75. You could
> even get fancy and use a motor driven cap and switch it between
> antennas, although retuning with band changes is a pain.
>
> OPTION 3: Another option is to use only one L and feed it on 160, 80,
> and 40. This would require a matching network at the base. This network
> will be more complicated than a single capacitor. You could build a
> separate network for each band and switch them in with relays. You could
> also include networks for both 80 and 75. I wouldn't use the same length
> L as you have for 160 meters, because it will be very high impedance on
> 80 meters.  I would either shorten it lengthen it to avoid this very
> high impedance on 80 meters. On 40 meters this antenna will look more
> like a dipole than a vertical.  Maximum radiation will be parallel to
> the horizontal wire, but with more gain than a vertical.  Nulls wont be
> as big as with a dipole because the vertical wire will fill them in.
> Another option is to use a remote auto tuner if you can find one to
> handle the power you are running.
>
> If you can put up more than one wire, option 2 is the simpler choice.
> The radial system you have should work OK for any of these options.
>
> One other note.  If you like to use 75 ohm transmission lines, you can
> uses a 1/4 section of 50 ohm line to match these Ls. Without a matching
> network, if your equipment operates OK with SWR of 1.5 to 2, you may
> elect to not have a matching network, but then you can't tune the 80
> meter antenna between 75 and 80, or move the frequency of the 160 meter
> antenna.
>
> Jerry, K4SAV
>