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
Eric Rosenberg wrote:
>I have an inverted-L for 160 with 24 radials that works really well.
>
>I'd like to add 80m and possibly 40m by having those respective vertical
>elements attached to the same radial system.
>
>My question: how far apart must the respective elements -- vertical and
>horizontal -- be from each other?
>
>My concern is a result of there being a single tree (i.e., tie off point)
>at each end of the antenna (where the L takes place and at the end of the
>horizontal section).
>
>Your help and advice is greatly appreciated!
>
>Eric W3DQ
>Washington, DC
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