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 ???

"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 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

"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

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

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

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
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