[TowerTalk] Improving the 402-CD
Sat, 1 Sep 2001 19:58:54 -0400 (EDT)
>> I plan to use large 4'5" diameter coils (sic)
> I don't think the elements will support 4 FOOT 5 INCH
> diameter coils ! N4KG :-)
Ooops - hopefully the intended 4.5" diameter was obvious! :o
>> CC 402-CDs work fairly well, but they can be improved.
> How far below a FULL SIZE 2L Yagi is a 402CD
> in gain? Maybe 1 dB?
> How much improvement can you expect from
> a shortened (loaded) Yagi, even with Lossless coils?
> My guess is you might get close to 1 dB more gain
> but you will still have limited bandwidth due to loading.
True. Models of the new design indicate up to 1.2 dB more gain
with a lower feedpoint resistance on CW which is matched to 50
ohms using a shunt inductor. With no shunt inductor, the
match on SSB is quite good. Using a relay to switch the shunt L
in or out, SWR is < 1.5:1 over nearly the entire 40m band so full
CW/SSB bandwidth is available at the flip of a switch.
> Have you considered plating the coils?
That might be a good option - I'll look into it.
> Note that F/B and GAIN typically do NOT peak with
> the same tuning / spacing of the elements. To achieve
> peak F/B, gain is usually reduced by around 1 dB.
True for full size antennas, however loaded antennas have a few
more parameters (i.e. size & placement of the loading) to help
adjust the peak gain and F/B frequencies. Models of the new
design show max F/B at 7.035 with gain peaking at 6.985. The
gain peak was left lower in frequency to obtain better SWR
> If F/B is a major concern, the best solutions are to either
> stack a pair of 2L Yagi's or go to a 3L Yagi, optimized
> for F/B (typically 7.0 - 7.5 dBi free space gain) on a
> 35 ft boom for 40M. Going to a 48 ft boom (0.35 WL)
> will yield another dB of gain at somewhat less F/B.
> Tom N4KG
My original plan was a 402-CD stack at 140'/70' but after buying
the AO software, I decided to tweak the design. Larger antennas
will not turn at the 70' level due to guy wires. A full-size 3L
on top would surely be a better performer, but that is a much
bigger and more expensive undertaking than relatively minor
changes to a 402-CD. A full size 3L sure would be nice though -
perhaps in another few years....
Steve Miller N8SM firstname.lastname@example.org http://www.dma.org/~millersg
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