Date: Thu, 02 Sep 2010 21:33:37 -0700
From: "Jim Brown" <jim@audiosystemsgroup.com>
Subject: Re: [TowerTalk] Testing a vacuum variable Cap for an Inv-L
I said:
>>A length of good quality RG8 would do that job!
>### Be real careful when using coax as a capacitor. It's lossy
>at best... and doesn't make for the best of capacitors... but will
>work in a pinch.
That's an old wives' tale. There is VERY little dielectric loss in decent coax
at 2
MHz. Heck -- I used 10 ft of Commscope 3227 to resonate a transformer at the
high Z
feed point of an 80M half wave vertical. No problem at all, and that was a VERY
HIGH
VOLTAGE POINT.
&&& 95% of coax loss is dielectric loss. It might work on 2 mhz... but has
been
problematic on 20-10m. When coax is used for a gamma match on 20-10m...
they sometimes don't fare to well. Coax cable also makes for lossy traps, if
used
to make coaxial traps. It would prob be ok on 160m in some applications..but
would
not be my 1st choice.
>## disc ceramics are not noted for handling high RF currents, esp on 160m...
Another old wives tale. Some do, some don't. You have to study the specs. OR,
you can
try them in an antenna. That's what I did -- I bought a bunch of HV caps (3kV,
6kV) at
the local electronics surplus store, stuck them in series with the antenna,
transmitted
at 1.5kW for a while, then turned off the TX and went out to feel the caps.
Some ran
hot, some ran cool as a cucumber.
&&& I have yet to find any specs on any brand of disc ceramic cap. They
only
list temp coeff... [which is usually lousy]... and peak V..and value. If you
have
access to box loads of surplus caps in the 3+ KV variety, by all means, try
them.
By the time you run outside, they will have cooled off too much. For a more
accurate test, leave the TX on with 1.5 kw cxr.... then go out there... and use
a fluke 62 mini... and point it at the caps..from 1" to 2" away. 2 kv rated
disc
ceramics appear to handle more RF than 1 kv rated units. You can buy
Ceramite
brand disc ceramics in 10/15 kv as well [mouser].
>Any cap will have to be either NPO.. or N470. You don't want it to drift
>about.
Another old wives tale. Temperature coefficient only matters if it gets hot. :)
The
good caps don't get hot, so TC doesn't matter.
&&& Nonsense. Temp Coeff is everything in any tuned circuit. Temp
coeff is not a big deal for caps..only when they are used for bypass..or
coupling.
&& in the case of a 3/8 wave vert...the cap is not only coupling....it's also
used
to provide XC to tune out the XL of the ant. A lot of disc ceramics have lousy
temp coeff.. and don't take much of a temp rise at all..over ambient to make em
drift.
They will drift downward in value..since they are all have a neg TC. Vac caps
have
superb TC's.
>Or use a parallel combo that will easily handle the current involved..abt 3-4
>x over.
Yes, caps in parallel are a good, inexpensive solution.
## both disc ceramic caps and disc ceramic TX doorknobs will not
handle any where near as much current on 2 mhz.. as they will on 30 mhz.
TX doorknobs are rated at 1-10-30 mhz.
> With the 2500 pf motorized vac cap the swr is dead flat across the entire 1.8
> to 2.0
> mhz range. Motor driven vac caps are ideal for this application. They
> don't drift,
> they handle loads of peak V, and they handle loads of current.
They are sexy, high tech, and VERY expensive. I have lots of other things to do
with my
money. Low cost surplus disc caps work just fine. Some of us are obsessed with
VSWR. I
am not. As long as your VSWR is less than 2:1, the additional loss due to SWR
is less
than 0.1dB. It is EASY to get a simple vertical to be flat over 160 M -- simply
double
up on the wire, with spacers between the two wires. Cost is under $10 for the
wire and
spacers.
&&& If you buy vac caps NEW... yes, they are cost prohibitive. They are
more than reasonable on the used /surplus market..and will last a lifetime..and
have excellent resale value. If you use disc ceramics...you will require a
box of
em to get the right combo that works. Change anything on the ant... and you need
to change the cap value..pita. Plan B is to use a broadcast variable... and
tweak
it for flat swr..then measure the value with a LCR meter.... then
use a combo of disc caps that will work. In some cases, a 2 or 4 wire cage
is not feasible due to weight, windload, ice, heavy wet snow etc. A 2:1 swr
just means you now have to constantly re-tune the linear. I design the linears
so they are quite a bit more broadbanded... [ into a dummy load]. A rising
swr
is a pita..when all you have to do is push one button..and flatten it..in less
than 2 secs.
Lots of hams waste lots of money on the basis of old wives tales. There ARE
good uses
for vacuum capacitors, but the feedpoint of a low impedance antenna near
resonance is
not one of them.
&&& depends on whether you have access to one at a reasonable price or not.
Another method that works is a relay tapped coil. Surplus vac latching relays
are a dime a doz right now..and work very well for this application.
A motor driven compressible coil will also work. All 3 of thee schemes work
very well on a 80 or 40m yagi.
later... Jim VE7RF
73, Jim K9YC
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