I haven't got the room for a good 80m antenna. My vertical covers 40 so no
trouble there. I am looking for ideas on a short 80m that will give half
decent results. I have about 66' available straight or can make some kind of
inverted vee ???

MD wrote:
I haven't got the room for a good 80m antenna. My vertical covers 40 so no
trouble there. I am looking for ideas on a short 80m that will give half
decent results. I have about 66' available straight or can make some kind of
inverted vee ???

My suggestion: Use the 102 ft G5RV length and let 18 ft. hang down
on each end. Feed it with ~23 ft of 450 ohm ladder-line. Put a
parallel cap of ~1000 pf across the ladder-line at the ladder-line
to coax junction on the antenna side of the choke. You probably won't
even need a tuner for the resonant frequency.

My second choice would be to go with the 66 ft. total length with
high-Q loading coils in the center of each 33 ft. leg.
--
73, Cecil http://www.qsl.net/w5dxp

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"MD" wrote in message
...
I haven't got the room for a good 80m antenna. My vertical covers 40 so no
trouble there. I am looking for ideas on a short 80m that will give half
decent results. I have about 66' available straight or can make some kind
of
inverted vee ???

Put up 66 ft of wire, or any length that is convenient. Wire length is
unimportant. Inverted vee is ok. Feed it with open wire line (Or 450 ohm
ladder line) an any point you like. It will work just the same as a full
sized dipole. Do not use any coax in the feedline, losses can be very high,
even with very short lengths. You do need a well designed tuner, with low
losses. Fairly high voltages will be present at the output of the tuner.

Regards.

Frank

Put up 66 ft of wire, or any length that is convenient. Wire length is
unimportant. Inverted vee is ok. Feed it with open wire line (Or 450 ohm
ladder line) an any point you like. It will work just the same as a full
sized dipole. Do not use any coax in the feedline, losses can be very
high, even with very short lengths. You do need a well designed tuner,
with low losses. Fairly high voltages will be present at the output of
the tuner.

Regards.

Frank

As an example on 3.8 MHz: 66 ft of #14 AWG wire, fed in the center, 30 ft
high above average ground Er = 13 Sigma = 5 mS/m. Radiation efficiency
96%. Fed with 50 ft of 600 ohm open wire line: input impedance = 748 +
j2087 maximum voltage at feedpoint, with 1.5 kW input, 3.05 kV. Series C,
shunt L tuner, with inductor loaded Q of 200. Tuner Loss = 0.24 dB.

Frank

Can you tell us what you want to do with it?

MD wrote:
I haven't got the room for a good 80m antenna. My vertical covers 40 so no
trouble there. I am looking for ideas on a short 80m that will give half
decent results. I have about 66' available straight or can make some kind of
inverted vee ???

As an example on 3.8 MHz: 66 ft of #14 AWG wire, fed in the center, 30 ft
high above average ground Er = 13 Sigma = 5 mS/m. Radiation efficiency
96%. Fed with 50 ft of 600 ohm open wire line: input impedance = 748 +
j2087 maximum voltage at feedpoint, with 1.5 kW input, 3.05 kV. Series
C, shunt L tuner, with inductor loaded Q of 200. Tuner Loss = 0.24 dB.

Frank

Forgot to mention. The transmission line loss is about 0.2 dB.

"chuck" wrote in message
news
Can you tell us what you want to do with it?

I have an R7 up for 40-10. Two maple trees on the property, one in the front
yard and one in the back but not far enough apart for an 80m dipole. The
trees runs east west over the house. I would prefer to run the dipole north
south, using the backyard maple for a support. The lot at this point is 66'
wide. One other thing to mention I don't think I can run ladder line or open
wire feeder into the house. I am pretty much limited to coax.

Thanks for the help,

Max

Frank wrote:
Put up 66 ft of wire, or any length that is convenient. Wire length is
unimportant. Inverted vee is ok. Feed it with open wire line (Or 450 ohm
ladder line) an any point you like. It will work just the same as a full
sized dipole.

EZNEC says that a 66 ft dipole used on 3.8 MHz, fed with 450 ohm
ladder-line, will have an SWR of greater than 100:1. This can
lead to all sorts of undesirable effects including an almost
impossible to match impedance at the tuner. A practical rule of
thumb might be in order, e.g. mine = no more than 20:1 SWR on the
ladder-line.
--
73, Cecil http://www.qsl.net/w5dxp

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I guess my question wasn't clear, Max.

Even though your space is limited, the type of antenna you install
should be informed if possible by the type of operating you intend to
do. If DX is very important to you (and here, DX could mean one or two
thousand miles) you might want to work in some vertically polarized
radiation, such as with an inverted L or a T (vertical with capacitive
element on top) or a sloper. How tall are those maple trees?

It may even be possible to load the R7 on 80 with a switched loading
coil at the base. Wouldn't win any efficiency awards I'm sure. But
thinking about it is pretty cheap and even fun.

Chuck

MD wrote:
"chuck" wrote in message
news
Can you tell us what you want to do with it?


I have an R7 up for 40-10. Two maple trees on the property, one in the front
yard and one in the back but not far enough apart for an 80m dipole. The
trees runs east west over the house. I would prefer to run the dipole north
south, using the backyard maple for a support. The lot at this point is 66'
wide. One other thing to mention I don't think I can run ladder line or open
wire feeder into the house. I am pretty much limited to coax.

Thanks for the help,

Max

I have an R7 up for 40-10. Two maple trees on the property, one in the
front yard and one in the back but not far enough apart for an 80m
dipole. The trees runs east west over the house. I would prefer to run
the dipole north south, using the backyard maple for a support. The
lot at this point is 66' wide. One other thing to mention I don't
think I can run ladder line or open wire feeder into the house. I am
pretty much limited to coax.

Thanks for the help,



I just got done building a "shortened" 75M dipole for traveling. It
is 27' on each leg. Homemade coils are out from the apex 15'. Doing a
little research in some old antenna books, and on web, I found good,
easy, information on calculating the value of the coils, depending on how
far out from the apex you want. In my case, I needed 50uH coils, which I
wound on some 1.5 inch plastic pluming material with #22 close spaced
telephone house wiring. For my little 5W FT-817 this works quite well,
but if you are running more power, you'll need to build a bit more robust
coil. Apex is held up with one of those 23' RV telescoping fibreglass
flag masts.

While open ladder line feed would give you a lot more flexibility, if
you must use coax, then what I did sure would be an easy thing to do. If
you need coil computation data and can't find it yourself, email me and I
will try to get it to you.


Ed K7AAT Email: Frog850 AT Bivalley.net