Folded Dipole
 

I have a spool of 300 ohm antenna feed line out in the barn,
never been used. Probably enough there for some kind of
antenna, I will have to unspool it and see how much is there.
Anyone have anything good to say about those folded dipoles
that use this 300 ohm line for both the antenna and the feed
line? I have seen illustrations of folded dipoles using
this, but in actual practice this line will naturally have
some twising when it is suspended. I cannot imagine how you
would keep it from doing this to some extent. What effect
would this twisting have on the effectiveness of the antenna.
The illustrations always show them geometrically perfect and
straight and parallel.


Bill

On Fri, 16 Apr 2004 04:15:57 GMT, zeno wrote:
Anyone have anything good to say about those folded dipoles
that use this 300 ohm line for both the antenna and the feed
line?

Hi Bill,

Nope, not for both - hardly for either (and it is arguable that its Z
even comes close to 300 Ohms). No reason not to try either.

I have seen illustrations of folded dipoles using
this, but in actual practice this line will naturally have
some twising when it is suspended. I cannot imagine how you
would keep it from doing this to some extent. What effect
would this twisting have on the effectiveness of the antenna.

Keeps it from thrashing in the wind. As far as noise pick-up, the
twist (used by the power company) reduces that too. Insofar as its
intended purpose - no effect, detrimental or beneficial.

The illustrations always show them geometrically perfect and
straight and parallel.

Silly editors....

73's
Richard Clark, KB7QHC

On Fri, 16 Apr 2004 04:15:57 GMT, zeno wrote:

I have a spool of 300 ohm antenna feed line out in the barn,
never been used. Probably enough there for some kind of
antenna, I will have to unspool it and see how much is there.
Anyone have anything good to say about those folded dipoles
that use this 300 ohm line for both the antenna and the feed
line? I have seen illustrations of folded dipoles using
this, but in actual practice this line will naturally have
some twising when it is suspended. I cannot imagine how you
would keep it from doing this to some extent. What effect
would this twisting have on the effectiveness of the antenna.
The illustrations always show them geometrically perfect and
straight and parallel.


Bill

Hi Bill,

Used just such an antenna a long time ago, Worked great.. and was
cheap to make.. Problems that take some engineering :)

300 ohm line is not strong thus it will break either due to wind
action or smiply from weight of snow or ice if not well supported. .

you'll need a good method of supporting the connections at the center.

when wet resonant frequency will change.. lower if I remeber
correctly.

you'll most likely need a tuner -- or an old tube rig like we used
that wasn't that fussy about SWR.

Power limited to about 200watts.. safely. Less if not tuned exactly
or if line is wet.

there is not much advantage over an ordinary dipole.. it is for the
most part a single band antenna. But can be used like a dipole on 3rd
harmonic.

My recomendation put up a regular dipole , feed it with the 300 ohm
stuff and a good balanced tuner.. you'll get more milage that way.

73 Dave Kc1di

Bill,
If you like 'playing' with antennas, why not try it? I
doubt if you'll see any great difference in performance from
a 'normal' dipole, and matching the '300' ohm feed line will
be 'fun', but certainly 'do-able'.
The usual 300 ohm twin lead that you find has certain
problems when used for an antenna for the lower frequencies.
Mechanical problems, as in strength (which has already been
mentioned). Not really ~that~ big'a problem, but one you should
be aware of. Twisting the line won't cause any problems and may
relieve one, 'flopping' around in wind (also pointed out). Ever
seen one of the twisted 'streamers' used in advertising tear
it's
self apart? Well, very often, anyway?
Using a tuner (I know Cecil, but for me it's easier) to feed
the folded dipole will make things 'more sinple'
'Doc

Okay, I'm compelled to chime in on this one.

The ARRL antenna book has a "cookbook" design for using 300 ohm twinlead
to make folded dipoles. In my 18th edition its the first thing in
Chapter 15 "Portable Antennas", pg 15-1.

It uses an open stub of the same 300 ohm twinlead to match to a 50 ohm
coax. You could just as easily omit the stub and run the twinlead to a
tuner.

I have one made from Radio Shack 300 ohm wire. After cutting per the
cookbook, including the stub, it tunes up perfectly on 3.775, only 25 kc
up from the stated center frequency.

The drawbacks to the design:
1)The stub, which is 37 feet long for 80 meters. You can replace the
stub with a capacitor, but I had the twinlead on hand....and was
interested in the design.

2)The fragility of the twinlead. Dave, KC1DI, mentioned that in his post
too. I have the center taped to a foot-long chunk of electrical PVC, and
a bit of nylon cord helping to support the feedline drop. That's been
sufficient for two years, so far.

3)Power. I have no idea if this thing would stand up to a kilowatt, but
it works fine at 100 watts. I'd be afraid to "smoke test" it....

Twisting, flopping, and drooping seem to make no difference. It plays
as well as the inverted vee it replaced, but except for a better SWR
bandwidth, its just a dipole. With the stub match, and a 90 foot coax
run back to the shack, the SWR is under 2:1 from about 3625 to about
3910, but my rig's internal tuner handles the edges easily. The SWR
seems to rise more sharply on the higher end of the band. I don't
notice any changes in SWR when its wet--not yet anyway.

On the positive side, the antenna is relatively light, and you can drop
the 300 ohm feeder right to ground level rather than pulling 20 lbs of
coax 40 feet up. And if you have a spool of twinlead sitting
around--its free.

I keep mine because its already in place, and it works. When it falls
out of the tree, I might follow Dave's advice and put up a regular
dipole fed with the twinlead.

Its only unique quality is a slightly wider SWR bandwidth.

Have fun..

73
Keith, W4KAZ

Dunno about a folded dipole but some friends swear by using it to make
VHF/UHF J-Poles. They can be rolled up to carry easily or put inside plastic
water pipe for rigidity. An 80 meter J-Pole? Well ....???
73, K3DWW

"zeno" wrote in message ...
I have a spool of 300 ohm antenna feed line out in the barn,......