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.

Fact is Cecil I never pay any attention to VSWR, just complex numbers.
Anyway your comments made me re-analyze the problem, and I realize I made an
error in the transmission line and antenna tuner analysis. I thought the
transmission line loss I came up with was a bit low. The final numbers are
shown below. See if you agree with me; then explain where I went wrong, and
why the match will not work.

66 ft dipole, 30 ft high, #14 AWG: Input Z = 11.3 - j961 ohms.
50 ft of 600 ohm line: Input Z = 5.48 + j189.85 ohms, loss = 1.95 dB
Matching network: Shunt C = 296 pF, Series L = 22.8 uH.
(Obviously half the L for each side of a balanced.line.)
Max line voltage: 1.5 kW in = 3 kV.
Tuner loss: 0.44 dB.

Incidentally have you ever looked at a typical airport NDB site? For
example a 45 ft monopole on 350 kHz has an input impedance of 0.2 - j7054,
which is a VSWR of 4.9 million : 1 -- whatever that means -- virtually
touching the edge of the Smith Chart. I have even seen 5 mile approach NDBs
with 30 ft monopoles. Marine installations for 400 to 500 kHz operation
frequently had electrically very small inverted "L" antennas. Of course 5
S/m sure helped, but the losses in the tuners must have been significant.

73,

Frank

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


Did that for a number of years on 80 CW ........worked fine .....

73 and God Bless .....KI3R Tom Popovic Belle Vernon PA.

"chuck" wrote in message
ink.net...
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

The maple trees are pretty tall, getting up in there high enough would be a
challenge. I may try 66' per side and see how high in the tree I have to go
to get the angle out to the corners of the lot. Would a direct feed with 50
ohm coax be suitable with a choke balun? Mainly interested in local comms
out to 3 or 400 miles.

Max

On Mon, 15 Aug 2005 16:48:54 -0400, "MD" wrote:


"chuck" wrote in message
ink.net...
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.

Max, is that to mean you want to feed the 66' dipole with 50 ohm coax
(all the way from the rig to the centre of the dipole)?

Assuming the feedpoint impedance that others have modelled is about
right, and that was 11-j961 at 3.8MHz... the losses in 20m (60') of
RG213 would be around 18dB. That is, less than 2% of the power into
the cable reaches the feedpoint.

That isn't the end of the story, there will be tuner losses... but the
good news is that partly as a result of the very high coax losses, the
impedance presented to the tuner will be well within the range of
loads that can be transformed at fairly low loss.

So... less than 2% of the transmitter power reaching the feedpoint...
is that acceptable?

BTW, if you considered a shorter line, say half the length at 10m (or
30'), the losses would be around 13dB (note considerably more than
half of the losses for 20m of line - the loss/unit-length is not
constant along the line).

It turns out that in practice, it is very hard to get most of the
transmitter power to the feedpoint of a dipole when the dipole length
gets much below 35% of the wavelength. Using lossy feedline (like
coax) exacerbates the problem.

Owen
--

You can try bending the ends to fit in the space you have...something
like "[ " ought to work (top view--fed at the center, dipole). Or, if
you want to make loading coils, you can find the values you need in the
ARRL Antenna Book in the chapter on limited space antennas.

Scott
N0EDV

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 ???

Lots of good information here guys, I appreciate all the help. Currently I
am using a long wire elevated at the ghastly height of 15' using the eaves
of the house as a counterpoise. I can work stations out to ~ 400-500 kms,
maybe that is the best I can manage.

Once again, thanks for all the help.

de VE3TMT

"MD" wrote in message
...
Lots of good information here guys, I appreciate all the help. Currently I
am using a long wire elevated at the ghastly height of 15' using the eaves
of the house as a counterpoise. I can work stations out to ~ 400-500 kms,
maybe that is the best I can manage.

Once again, thanks for all the help.

de VE3TMT

I failed to notice you did not want to use open wire line. If you use
inductive loading, for example: nominal 46 uH, Q = 200, at 15 ft from each
end of a 66 ft dipole. The radiation efficiency is 71%, i.e you loose 1.5
dB in the loading coils. The input impedance is 28 ohms, and the losses on
60 ft of RG213 will only be 0.24 dB. Orientation of the dipole is
unimportant since at 30 ft high the radiation is essentially
omni-directional.

73,

Frank

"Frank" wrote in message
news:X2lMe.197169$tt5.130324@edtnps90...
I failed to notice you did not want to use open wire line. If you use
inductive loading, for example: nominal 46 uH, Q = 200, at 15 ft from each
end of a 66 ft dipole. The radiation efficiency is 71%, i.e you loose 1.5
dB in the loading coils. The input impedance is 28 ohms, and the losses
on
60 ft of RG213 will only be 0.24 dB. Orientation of the dipole is
unimportant since at 30 ft high the radiation is essentially
omni-directional.

73,

Frank

Hi Frank,

Unfortunately using open-wire is not an option, as I have to feed the
feedline in through the exterior brick wall of the house. I have 2 feedlines
running out now. One to the R7 and the other for experimenting. I am going
to try your suggestions with the loading coils, as soon as I get back from
vacation!!

MD wrote:
"Frank" wrote in message
news:X2lMe.197169$tt5.130324@edtnps90...

I failed to notice you did not want to use open wire line. If you use
inductive loading, for example: nominal 46 uH, Q = 200, at 15 ft from each
end of a 66 ft dipole. The radiation efficiency is 71%, i.e you loose 1.5
dB in the loading coils. The input impedance is 28 ohms, and the losses

on

60 ft of RG213 will only be 0.24 dB. Orientation of the dipole is
unimportant since at 30 ft high the radiation is essentially
omni-directional.

73,

Frank


Hi Frank,

Unfortunately using open-wire is not an option, as I have to feed the
feedline in through the exterior brick wall of the house. I have 2 feedlines
running out now. One to the R7 and the other for experimenting. I am going
to try your suggestions with the loading coils, as soon as I get back from
vacation!!

Can you get something through a window? If the window can be opened,
you can place plexiglas or wood in the window, and then cut the
appropriate openings for ladder line.

- Mike KB3EIA -