feeding an 80 meter loop question
VK3XL
Hello all, I am planning to erect an 80 meter full wave loop antenna in
the back yard. The articals I have read so far all seem to feed the
antenna at a corner with either 450 ohm open wire line or feed direct
with 50 ohm coax. Due to the layout of my yard it would be preferable
that I feed the antenna at the mid point of one side of the antenna
with 450 ohm open wire line to a tuner (4:1 balun included in the line
and coax to the tuner). Is there any disadvantage in feeding the loop
mid way along one leg rather than at a corner?
any assistance greatly appreciated
Mike
VK3XL
--
VK3XL
lt...@yahoo.com
Is there a disadvantage? Not really, but maybe. Depending on the
shape of the loop, it can/will change it's radiation pattern and
polarization. That may not be of any importance at all, won't know
till you try it.
- 'Doc
Wimpie
Hello Mike,
Nothing to worry about. Feeding at a corner or feeding in the middle
of the edge/leg has insignificant influence on radiation pattern (it
will be very good for NVIS operation).
The input impedance remains low (<<450 Ohm). Feeding with coax is
possible. Add a common mode choke to reduce influence of cable on
radiation pattern and local noise pick up. I assume that your loop is
almost square (so occupies large area).
Best regards,
Wim
PA3DJS
www.tetech.nl
the mail address is valid when you remove abc.
n4lq
the loop is fed. There are too many variables to predict this without
computer modeling. You would be better off using 300 ohm line instead of
450. 300 ohm would more closely match the impedance of the loop on higher
bands and result in lower swr excursions but either type of feeder will work
great. The obvious advantage to corner feed is less sagging but your feeder
is lightweight.
73
--
Steve
"VK3XL" <VK3XL....@radiobanter.com> wrote in message
news:VK3XL....@radiobanter.com...
Frank
For a loop, 20 m per side, 10 m above an average ground, the input
impedance is about 100 ohms (at 3.825 MHz). Feeding at the
corner, or the center, makes no difference to the input impedance.
The radiation pattern will rotate by 45 degrees. Very good high
angle radiation. At a 30 degree elevation angle the pattern is
like a dipole, with nulls about 4 dB deep, and max gain - 4 dbi.
73,
Frank
(VE6CB)
Edwin Johnson
>
> with 50 ohm coax. Due to the layout of my yard it would be preferable
> that I feed the antenna at the mid point of one side of the antenna
> with 450 ohm open wire line to a tuner (4:1 balun included in the line
> and coax to the tuner). Is there any disadvantage in feeding the loop
> mid way along one leg rather than at a corner?
Your radiation pattern will probably change somewhat, perhaps to the better
for what directions you desire. If you can model the antenna with software,
this will give you somewhat of an indication.
Just for information, I have a horizontal 80m delta loop fed with 450 ohm
ladder line which goes to a ferrite bead balun (W2DU style, built from a kit
from Wireman) to go from balanced to unbalanced and a short (less than 6'
coax) into the house to the tuner, an MFJ 941E. It tunes all bands 80m up
through 10m very well and performs excellently. Not the greatest DX antenna,
of course, but serves me well. Notice, this is a delta (three cornered) loop
and not four sides, due to yard size and tree locations. I feed my loop
about mid-side for convenience of feedline house entry.
73 ...Edwin, KD5ZLB
--
____________________________________________________________
"Once you have flown, you will walk the earth with your eyes
turned skyward, for there you have been, there you long to
return."-da Vinci http://bellsouthpwp2.net/e/d/edwinljohnson
Mike Luther
I used a three element 4-square 'loop' for years here. If your loop can or
will be a vertical loop, even though the lower side of the square will be
parallel to the earth and low to it, I can offer you suggestions.
Do *NOT attempt to feed it in horizontal polarization. You may go up midway on
either side of the vertical wire elements. On one of them. all you have to do
is attach the braid of the coax to the center point of the wire loop. Then,
feed the loop with a gamma match segment made from, say six inch balanced line
insulators which bind to the wire of the loop at the far end to the gamma line
and through a variable capacitor to the center of the coax.
You tune the gamma match for proper SWR and so on and away you go with a really
decent feed line operation to your rig. You can swing the coax to either the
center support pole for it all, or move it perpendicular to the loop away from
it in any direction off the the rig site. I believe you will be VERY pleased
with the performance of the loop for DX purposes as the angle will favor
vertical polarization, but you do not need ground support wires like in a
vertical.
There are a number of references on how to more or less figure out the length
of the gamma section.
Then, after you figure out the general value of the capacitor you need, you can
pull off another stunt! You can use another chunk of your coax cable as a
substitute for that capacitor! It has so much capacity for so many inches or
feet. OK, you use the outer braid for one lead of you 'capacitor' and the
inner wire for the other. You start with a little longer than needed or too
much capacitance, then start snipping off the coax at the free end of this
contraption to lower the capacitance until it is just right.
At the free end, be sure to leave an inch or so of complete coax insulation at
the tip away out from the remaining braid. Then wrap it if you like with tape
to keep out the water and so on. Zippee! Fixed very durable capacitor for
this gamma feed device.
I've used this coax into capacitor technique for decades now on gamma matched
verticals and so on. Never a problem ever with it.
The trick for getting low angle performance from large wire devices like loops
on 40 and 80 is to get them to really be in vertical mode. And not dependent
on the ground below for RF current return like in verticals that are ground
plane devices.
But to do that you'll have to make the loop in a vertical mode, which, unless
you choose to use loading coils to fudge the vertical height at the tips of the
wire so you can keep the feed point balanced and a little lower, will require
some kind of high support for the 66 foot or so vertical side of the loop at
two ends of it.
I can assure you the fixed wire quads on 40 meters built this way have a VERY
good signal and beam operation from personal experience. And their DX
performance goes WAY up hotter if you feed them in vertical mode rather than in
horizontal more the simple way down close to the ground.
--
--> Sleep well; OS2's still awake! ;)
Mike Luther
VK3XL
Thanks all who have replied to my question. I am glad to hear that there
are no serious detrimental effects of feeding the antenna mid way along
one side rather than at a corner. I should have mentioned that I am
intending to make a horizontal loop antenna rather than a vertical.
Another interesting point came out of this, a delta loop may provide a
better fit in the space available when the tree locations are taken
into account. Again thanks for the info
73 de Mike VK3XL
--
VK3XL
co_f...@yahoo.com
Hi, Mike.
I have been using a 160 meter, horizontal, almost square loop since
January. It is suspended from four 36 foot push-up masts at about 33
feet. It is fed at a corner with 125 ft. of home made 600 ohm feed
line. Works on all bands with an antenna tuner. Even made a couple of
local 6 meter contacts.
The challenge is to get each of the 4 wire legs under the same
tension. The connection of the antenna wire to the corner insulators
must be able to move so you can get it all evened up. Once up, the
back stay at the top of each mast, along with the 90 degree antenna
wire make for a very stable installation. The fierce winds we
sometimes have in Central Oregon hardly move the antenna wire.
I made the corner connectors out of a circular piece of brass by
milling a circular slot about 1 inch away from the center of the
circle. Then cut the circle into 4 sectors. I machined 4 brass covers
to fit over the slot after the antenna wire is placed in the slot. The
covers are held by 4 small socket head cap screws. Anti-seize compound
protects the threads from rust. A hole is drilled in the back of each
brass corner piece to connect to the ceramic insulator. I cut lengths
of heavy copper wire and after putting it through the insulator and
brass corner piece, pounded the ends flat. The heavy wire was formed
into a loop with overlapping ends. The heavy wire ends were then
drilled to accept a small machine screw and nut to hold the ends of
the copper wire loop.
The top of each push up mast has a pulley and 1/4 inch rope to pull up
the antenna corner. Several trial runs were made get it all up in the
air and get the tension pretty much equal.
I think you will like your antenna a lot. Good luck with it.
73's Paul, KD7HB