Last week I built the fan dipole from
http://www.hamuniverse.com/multidipole.html for 80/40/20, figuring I
would use the 40 on 15.
I used 3/8" square 4' long PVC gray rod (McMaster) cut to 3'+ long to
separate the dipole ends 3' and a simple Cu bus on 1/4" thick acrylic
for 5" of center separation. The feed is with 121.5' of Buryflex (like
9913-F7) and a Cal-Av bead balun.
I used an AIM4170 to measure the antenna/coax at the rig and also set
the calibration plane to the feedpoint to measure the antenna w/o coax
Resonance is a little higher than I'd like (mid band) on 80m,
substantially high on 40m and ok on 20m with the calculated dimensions.
Some numbers from the AIM4170 plots:
band / resonance / Z ohms /2:1 BW Khz / 3:1 BW Khz
80 /3.78 /65.1 /220 /300
40 /7.24 /73.6 /200 />350
20 /14.1 /50.0 />350 /lots
The antenna has exceeded my expectations and performs better at 45' than
my ladder line fed 80m doublet at 60', so I am building a second fan for
a different orientation. I think there are some advantages for the fan
vs the ladder line doublet, easy to overlap patterns if you can put up 2
of them, high confidence in balance with a decent choke balun, and no
problems routing the feedline or with weather changes.
With the center feed "bus" its a little hard to guess what the actual
electrical lengths are, so here are the wire quarter wave lengths for
version 2, now in construction:
80m 63'10" for 3.65 vs 3.78
40m 33'6" for 7.1 vs 7.24
20m 16'4" for 14.2 vs 14.1
Even with low loss coax there is enough loss in 121.5' to have the WARC
and 10m bands SWR within range of most tuners, if you don't mind the losses.
Bottom line, even with the article's promise of a "no tweaks" design,
you will likely need to prune to get what you want, but I think there is
much less interaction with the dipole ends separated by 3' and the
feedpoints separated by 5".
b.t.w. I've measured the Cal-Av balun vs a two core FT240-31 balun with
8 turns RG-142 TFE coax as recommended by Jim K9YC. I was too lazy to
build the box etc for the toroids for the fan but toroid balun Z
measured about 2.5x the Cal-Av. Stacked type 31 toroids win big for the
low bands vs many many beads.
On 4/20/2011 7:03 PM, K8RI on TT wrote:
> There are multi band dipoles such as fan dipoles, trap dipoles, stub
> detuned dipoles, and linearly loaded dipoles.
> It's been mentioned that with fan dipoles the lowest frequency band has
> it's band width increased, but on the higher frequency of say 160 and 75
> we find the band width narrows. Remember this is also the second
> harmonic of the 160 segment and the bands tend to be harmonically related.
> Is it this function that tends to cause the higher bands to lose usable
> band width?
> Some swear the praises of trap antennas, but they are relatively
> inefficient compared to others and it takes some beefy traps to handle
> the legal limit. These traps can see some pretty high voltages at the
> legal limit and when used with a tuner these traps can see some even
> higher voltages.
> I'd think stub tuning, or decoupling would be easier to do with ladder
> line than the "fan" approach of parallel wires.
> I really don't know much at all about the linear loading aspect.
> Then there is the *balun* at the feed point. Most of the commercial
> multi band wire antennas use a voltage balun at the feed point and for
> 75 and even 40. Many rigs require a tuner to cover the whole band. Most
> of these baluns are incapable of handling the legal limit with much of
> an SWR and a tuner will allow the balun to get hit with more power,
> generally cooking it when operated outside it's SWR limits. I think many
> of these are pushed to handled the legal limit PEP, let alone higher
> duty cycle modes even with a low SWR.
> There is a lot of equipment out there that is generously rated even when
> it comes to PEP, let alone digital modes.
> Roger (K8RI)
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