[TowerTalk] Improving the 402-CD

Steve Miller millersg@dmapub.dma.org
Sat, 1 Sep 2001 19:58:54 -0400 (EDT)

N4KG writes:
><millersg@dmapub.dma.org> writes:
>> 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   millersg@dma.org   http://www.dma.org/~millersg

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