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WA2ISE > TECH 04.12.20 21:05l 60 Lines 2873 Bytes #999 (0) @ WW
BID : 2209_WA2ISE
Read: GUEST
Subj: Twinlead folded dipole for 2 meters
Path: IW8PGT<IZ3LSV<F6BVP<F3KT<CX2SA<N3HYM<AL0Y
Sent: 201204/1903Z 10376@AL0Y.#NNJ.NJ.USA.NOAM BPQ6.0.20
Twinlead folded dipole for 2 meters
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|_______________________ _________________________| 300 ohm twinlead
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balun
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50 ohm coax
This antenna has a much wider bandwidth than a Jpole, and is a little smaller
too. And a decent SWR. Better than 1:1.3 over the entire 2M band, 1:17 in the middle
of the band. Good for packing for a business trip where there are
repeaters around. The antenna is 97cm long, but cut the twinlead a little longer
(like a cm longer) so you can short the pair of conductors in the twinlead at the
end together (and likewise the other end). At the midpoint cut only one side of
the twinlead. Strip a little length the wire, to get a gap equal to the 300 ohm
cable on the TV balun. Trim off any lugs off the balun, and solder these balun
leads to the midpoint wires you just prepared. TV balun? Yes, it's still a 4:1
balun, was 300 to 75 ohm, here it now is 200 to 50 ohm, to match some RG58 coax to
feed your 2m HT. Use an F to BNC adaptor. These baluns can handle around 10 watts
no problem. Another reason for the balun is to avoid common mode RF current,
in which the RF current on the outside of the coax affects the antenna pattern.
If you want, you could create a balun out of ethernet twisted pair. Use 6 turns of
one twisted pair over a coil form 5mm diameter. Make two of these. The turns are
the pairs of "3's" here:
| to twinleas |
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| ________ |
3 3 3 3
3 3 3 3
3 3 3 3
3 3 3 3
| | | |
| -----------
----------- |
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gnd coax 50 ohm
A twisted pair from a CAT5 cable has 100âä¦ impedance. The twisted pair, or the
bifilar pair, acts like a transmission line, delivering the same impedance at the
source, at the far end. The coiling makes it "forget" (at the RF frequency of
interest) at the end that one side of the twisted pair was grounded at the source. At
the coax connector, the two coils are connected in parallel, the two 100âä¦ impedances
looking like 50âä¦ impedance to the HT.
Using a Rigexpert antenna analyzer, I trimmed the ends to get the antenna resonant
in the 2m band. Be sure to have the feedline perpendicular to the antenna for a
minimum of ¼ wavelength and more is better (I used some string to hang this antenna
up away from stuff) to get valid measurements and good performance. Or use suction cup
hooks and string on the glass of the sliding balcony door to hold the antenna and
feedline. With an HT, it hits my test repeater at 146.955 well.
73 de WA2ISE
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