Folded Diapol Antenna with Twin Lead Antenna Wire
Would the closness of the 300 ohm TV twin leads wire be a problem if used in a half or full length diapol antenn cut for a pacific band? If not, would other bands be workable too? Also, sence I am considering a full length (1/2 wave/leg) antenna, would that affect the coax match connection/balum needed? I am a newbe & full time RVer & could use this approach to simplefy my use at campgrounds! However, hanging height would not be great! 73, Julius KF5BBB
Last edited by KF5BBB; 12-25-2011 at 11:28 PM.
I didn't really understand the question, but I "think" you're talking about a 1WL doublet, rather than the conventional 1/2WL doublet. There's a huge difference.
What band(s) are you talking about, and what lengths?
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-- George Bernard Shaw
The 300Ω twinlead is a bit light for that antenna. You could use regular
# 12, stranded copper for the antenna and connect it to 450Ω or even
600Ω stranded, twin-lead. You should find a place where you can let
some RG-8X, Super 8 out of the motor home to tie to the twin lead.
Then the OTHER end of the Super 8 goes to an antenna tuner like an
MFJ 949-E, if you run less than 300 watts.
Now, hanging the antenna could be made simplier buy using a fishing
rod with a cheap Zebco 202 reel and weight. Get the weight over the
highest tree limb, tie the antenna to the fishing line and haul it up.
Once it gets going, simply tie it off a tree, then cut the line. Then, go
do the other side. When you get ready to leave, just cut both fishing
lines. roll up the antenna and twin lead and motor on.
I've done this when we used to go camping for a week. Just make sure
the twin lead or coax is out of folks way.
There are some TALL pine trees in N.E. Texas and the 10 meter antenna
was up about 80' on both ends.
I've done it too with a coaxial dipole for 17 meters as well.
I have a 100 W transceiver that has a built-in antenna tuner. My reason for the 300 ohm TV twin lead wire for an antenna was it is light weight, inexpensive & already folded. I would use 2 loops with each either a 1/4 or 1/2 wave when as folded (made). I would solder the outside ends to make a loop on each. I believe I would need a 4 or 6 balum between the antenna & the coax lead-in to the motor home. I have a fishing rod & real to raise into my low trees (15'-20').
Do I understand that you recommend I use ladder type twin lead as it has heaver wire? OR would that give me better transceiver singles?
73, Julius KF5BBB
(could believe it when I got my call sign - Better Business Bureau - I once had my own business)
My very first antenna as a novice was a folded dipole made from 300 ohm twin lead and using 300 ohm twin lead as the transmission line as well. It is much stronger than you think, mine survived the Santa Ana winds just fine. A folded dipole that is one half wavelength long on 40 meters can also be used on 15 meters and will have a bit more bandwidth than a single wire dipole. A folded dipole does not have gain and operates exactly like a single wire dipole but with a higher feed point impedance and a broader SWR bandwidth.
Originally Posted by KF5BBB
A 4:1 balun can be placed between the 300 ohm feeder and you can run coax to the radio. The Balun can be inserted anywhere between the feed line and the antenna, even right at the antenna feed point but the weight of it will likely cause problems if you connect it directly to the antenna.
I'm sorry you don't have the experience or understanding to realize that others possess a skill set that you seem to dismiss as fantastical.
Balun ... aka BALanced to UNbalanced.
A dipole is a half wave antenna, not full wavelength.
Would it also work on 20M? I figure each leg when folded should be about 15' long which would get me a full wave for each leg or 40M. Does that sound right? 73, Julius, KF5BBB
Many years ago in the Radcom magazine there were details of how to make a 12/17 metres dipole from 300 ohm ribbon feeder. 17 metres supporting the antenna and the other wire cut shorter underneath, fed with RG58, so give it a try, it'll work
A dipole is a half wave antenna. The reason a 40M antenna works (fairly well) on 15M is it is an odd harmonic. But ... the antenna needs to be cut for the low end of 40M to make it work well on 15M.
A dipole for 20M works on 20M.
A dipole for 80M works on 80M. etc.
Using open wire line rather than coax lets you use an external tuner and make a longer antenna work on higher frequencies.
I *think* he is saying that the *total* amount of wire in a 1/2 wavelength *folded* dipole is 1 wavelength in feet. This idea that total feet of wire makes an antenna "match" is incorrect - whether we are talking about folded dipoles, or loading coils.
Yes, twinlead can be used.
We don't describe folded dipoles in terms of the total length of wire. You will confuse yourself and everyone here if you do that.
It is a half wavelength long - e.g. you cut a half wave length of twinlead, solder the two conductors together at each end, then cut one conductor of the twinlead exactly in the middle and attach the rest of your twinlead as a transmission line at that point. I think you already understand that.
A folded dipole cut for 20m will not work on 40m - regardless of the total length of wire. Don't confuse length of wire with impedance. They are not the same thing. (it would work as a half wave dipole on 40m ONLY if you sliced the twinlead down the middle, cut the top of the folded dipole, and "unfolded" the wires so you had two 1/4 wavelength legs) - but it would be a 50-75 ohm antenna, and not the ~300 ohm folded 1/2 wavelength 20m dipole at that point.
A (perhaps) better choice would be a dipole with single wire legs, fed with twinlead, through a balun to your tuner. You can experiment with lengths of wire. I used a dipole like this made of two 33' legs of 16ga stranded wire, fed with 50 feet of cheap 300 ohm TV line from the hardware store as a portable antenna, suspended as an inverted vee from a 31' fiberglass pole. I tuned it through an old MFJ-945C portable antenna tuner. It tuned easily for me on 40-10m. I was only running a maximum of 100 watts.
Don't fall into the trap of wire length = impedance. You will get very confused, very quickly if you think that way.
The same folded dipole of 300 or so ohms, if stretched out into a loop has yet another impedance - but it is a full wave loop, not a folded dipole. The orientation and spacing of the wires is what "makes" the impedance and current distribution on the antenna - NOT the length alone.
Last edited by K1DNR; 12-29-2011 at 04:27 AM.