[TowerTalk] Question on Multiple Inverted L Antennas

[K4SAV]

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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