Please be aware that the bandwidth for a coaxial inverted L is very broad. My
SWR at 1.800 is 1.3 and slowly rises
to 1.7 at 2.000Mhz. (The antenna is cut for 1.8400MHz and is physically
located about a foot away from the base
plate of one of my 80M coaxial inverted Ls). Because of my phasing system, It
is not easy for me to measure the
SWR curve from the shack for my 80M, 40M, or 20M coaxial inverted Ls, but I
remember that the curves were
decent for all of them when I installed them.
73
Bruce, WA3AFS
On 13 Jul 2006 at 13:11, K4SAV wrote:
> 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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