>
> The VNA might take the guesswork out of some of the measurements, but
> doesn't tell you much about how to change the design to make it
> "better". And for using "non-RF" components in RF applications
> (which is what most amateur market relay boxes are.. they are NOT
> clever stripline designs like DowKey uses) there's an awful lot of
> craft and art to the design, probably moreso than straightforward RF
> circuit design. (Unless someone's been out building HFSS models for
> relays).
The VNA is simply a tool. As with any tool it requires that the user have an
understanding of its abilities as well as his own.
The DowKey is not a stripline design, it is a coaxial line section; big
difference.
Commercial radio manufacturers have been using small open frame power relays
in RF use for over 60 years, nothing new there. Vacuum relays designed for
switching HV made an easy transition to RF.
>
> The VNA just tells you "good", "bad", or "about the same"... but
> doesn't tell you "make the box 1/4" taller so the parasitic C is
> less".. that comes from either mindnumbing analysis (probably a waste
> of time) or good old empiricism (try a bunch of different ideas, and
> see what works).
Ive never had parasitic C problems at HF in a straight 50-75 Ohm layout.
Amplifiers and even antenna tuners are another story. A lot of my work
resume included millimeter wave
circuitry up to and beyond 60 gHz; now THAT stuff is tricky! But you can do
a lot with
RF absorbent material at microwaves and above
After youve been around the circle a few times the old gut feeling is
often the key to success if I may borrow an anology from my younger
circle track racing days.
>
> And, a lot of what goes into amateur products (or pro products for
> that matter) is non-RF related.. manufacturing cost, durability,
> shipping, etc. all factor into component choices. I've seen
> products (not in the ham market, and not RF related) that were
> originally designed around the ready availability of a particular
> surplus component. Then, when the surplus supply ran out, the mfr
> had to go out and have custom parts made to duplicate a long
> discontinued WW-II vintage part. That drove the Bill of Materials
> (BOM) cost through the roof... especially for replacement parts for
> the original widgets, and drove a completely new design.
A lot of surplus parts from WW2 Ford and Willys built jeeps found their
way into Kaiser civilian models.
Heathkit started in business with WW2 surplus parts.
I got my SWLing and later ham start with surplus from Radio Row in NYC
back in the 50's.
>
>>When KQ2M operated my station 1987-95, I designed and built all of the
>>elaborate stack switching and elevation steering for a 4 stack of yagis on
>>20, 15 & 10 as well as the pattern switching for 160 & 80 antennas. Specs
>>were so tight that amp tuning never changed even when several relays and
>>networks were in the signal path. Performance was obvious in the contest
>>scores.
>
> Just out of curiosity.. did those relay boxes work that well in the
> initial design, or were there any design changes? Do you think you
> might have just "gotten lucky" in the design and component selection
> in the 80s?
>
> I've been casually measuring a variety of off the shelf relays over
> the past year or so, and there's huge variations between seemingly
> identical relay designs from multiple manufacturers.
>
I had already characterized numerous relays and had settled on new DPDT PB
with dust cover 24VDC
surplus units from All Electronics. They had more than adequate return loss
and isolation at 10M
and below. Plus they were cheap, I think $2 each!
And since they would be mounted in gutted CATV hardline boxes I liked the
dust cover to
minimize condensation here in NH.
Since elevation steering was part of the array the VNA was a key in
determining the
built in phase delay through the various selections. With that determined
it was then easy to
cut the phasing cables to exactly what was needed. Since I was using all 75
Ohm hard and soft line it
was extremely advantageous that the company I was working for had the 75 Ohm
option kit for the HP VNA.
In the air performance was just as expected, no luck involved I hope! I
could barely see the
difference in grid current at any stack setting. This kept the LK780 happy
(a very low production
2 x 3CX800A7 amp from Amp Supply) and the LK500 was always idiot proof.
(KQ2M was one of the
originators of the SO2R concept).
Remember that I didnt operate those contests and the first design goal was
to NOT be woken up at 3AM to fix a
sleepy operator induced problem!
When Bob got married and bought his own place in SW CT I dismantled most of
the hardware.
These days my interests are primarily 80/160 and then 6M and above; way
above.
Carl
KM1H
> Jim, W6RMK
>
>
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