Thanks LB for your wonderful posts on the folded dipole. It is truly an
efficient antenna for single and odd-harmonic operation, but not great as an
all band antenna. My experience has indicated that a properly tuned folded
dipole is slightly more efficient than a single wire dipole, but a single
wire dipole cut for 40M, fed with balanced feeders is a remarkable performer.
If I have room for only one antenna, that is my antenna of choice! I've used
it mostly in inverted vee fashion with thr apex between 25 and 35 feet, and
the ends between 8 and 10 feet above ground and have always been pleased with
the results. Even on 80M, I can tune it as a top loaded vertical by shorting
the feeders together and tuning it directly without the balun in place.
I agree completely with your observation that you do NOT consider velocity
factor when cutting the folded dipole to length, and I have a load of
experience top prove this. Here is my methodology for a no-tuner folded
dipole. 1) I design a 6:1 balun to provide a 1:1 SWR into a 300 ohm
resistive load (carbon resistor). 2) I extend my 300 ohm feedline out to the
feedpoint that I'm going to use. 3) I attach the 300 ohm resistor across the
far end of the feedline and again confirm that I have a 1:1 load (most 300
ohm line has impedence characteristics as advertised, but some is off) I
attach the folded dipole that is about 2 % longer than formula, raise it to
the desired height, and prune the ends until I show a 1:1 SWR. Observing SWR
the frequency is moved up and down the band gives you a very good idea where
resonance is and about how much you need to cut off. As you raise and lower
the ends, you will note the resonant frequency go up and down slightly.
Since I only operate CW, I cut my dipoles for resonance at 7040. I can get
1:1 SWR from 7000 to 7100 KHz, but can even operate the entire phone band
with an SWR of less than 1.5:1. I use a broadband ferrite rod balun, and
with careful design, can also get a very respectable SWR on 15M as well.
Again LB, you are right on with your observations. A folded dipole is an
efficient single band antenna, but a single wire dipole antenna is much more
flexible for multiband use!
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