[Mldxcc] HELP: Doublet - 80m / 40m fan dipole

[K6ST Barry Bettman]

Need some help from the NCCC Brain Trust, please.

Bill WB6CZG in Truckee had a 80m Doublet (dipole) 133' long running on all
hf bands from 80m-6m with his KX3 and Tuner for 7 years until the massive
snow storms took it down this past winter.

Today we are put up an 80m / 40m fan dipole - doublet  to run on all HF
bands 80m-6m.

Bill build a fan dipole with the top horizontal element being 80m based on
468 / 3.5 mhz and the bottom inverted-v  40m based on 468 / 7.2 mhz.  The
fan dipole is up about 80' high in the trees with the 80m orientation
East/West and the 40M orientation North South.

The  450 ohm  ladder line feedline is about 80' in length to Fan Dipole.
Then the  450 ohm  ladder line connects to a 1:1 balun , which goes to
approximately 15' of  50 ohm coax into the shack.

When checking the SWR on an MFJ-269 analyzer,

On 80m

The lowest swr on 80m is at 4.19 MHZ, SWR=2.2  Rs=24  Xs=40 Ohms=40

For 50 ohms, the lowest swr on 80m is at 4.3 MHZ, SWR=3.2  Rs=29  Xs=38
Ohms=50


On 40m

The lowest swr on 40m is at 6.96MHZ, SWR=1.7  Rs=85  Xs=12 Ohms=90

For 50 ohms, the lowest swr on 40m is at 6.81 MHZ, SWR=4.3  Rs=21  Xs=43
Ohms=50


We see from the analyzer measurements, we assume 80m seems short in length
and the 40ms seem long in length, based on a pure dipole  468/freq mhz
formula  to swr-frequency readings, although we are using 450 ohm
ladderline not 50 ohm coax

Questions:

1. Is there some kind of interaction between 80m and 40m?

2. Is the 1:1 balun the correct balun for this antenna configuration.

3. Is there an optimal length for the ladder line?

4. Is there something else we are missing?


Maybe this was all for nothing ... see below

On the other hand, ladder-line does not suffer from high losses at high
SWR, so may be effectively used to feed an antenna that may, at various
frequencies, present the feed-line with any SWR from 1:1 to ~10:1. So, with
ladder-line, you can completely forget about resonance and SWR, until you
get to the radio, where you use a tuner to make the match to 50 *j*0 ohms.
http://kv5r.com/ham-radio/ladder-line/

--


Friday we will be back at it in hopes that we can get the antenna fully
working for the IARU contest Saturday. Thanks for you help.





73,

Barry K6ST
https://twitter.com/barrybettman
http://www.linkedin.com/in/barrybettman
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