For a while I was cooking vacuum variables in my 160 meter Omega match
network as well as SO-239's es "N" female connectors...
I couldn't figure out what the problem was?.............................
So I checked the line from the shack to the network... It was a 1/4
quarter wavelength line...
The problem was due to a voltage/current node at the line termination......
This is a well known phenomenon in electromagnetic theory. I added about
20 feet to the line and the problem disappeared!
* /Standing waves/are waves of voltage and current which do not
propagate (i.e. they are stationary), but are the result of
interference between incident and reflected waves along a
transmission line.
* A/node/is a point on a standing wave of/minimum/amplitude.
* An/antinode/is a point on a standing wave of/maximum/amplitude.
* Standing waves can only exist in a transmission line when the
terminating impedance does not match the line’s characteristic
impedance. In a perfectly terminated line, there are no reflected
waves, and therefore no standing waves at all.
* At certain frequencies, the nodes and antinodes of standing waves
will correlate with the ends of a transmission line, resulting
in/resonance/.
* The lowest-frequency resonant point on a transmission line is
where the line is one quarter-wavelength long. Resonant points
exist at every harmonic (integer-multiple) frequency of the
fundamental (quarter-wavelength).
Since I fixed the problem the network tunes more smoothly. The 1000 pfd
vacuum variable to ground is tuned with a reversible 12 VDC, 1 RPM
motor. The other 1000 pfd vacuum variable is in series with the OMEGA
arm and is only adjustable locally....
73,
John, W4NU
K4JAG, 1959 to 1998
On 3/13/2017 12:41 PM, Guy Olinger wrote:
Once you know the problem is the cap, then you need to get really serious
about the cap. Caps in RF tuning networks are stressed applications.
Decide if you ever want to use it for anything except short and separated
cycles, like calling DX. If you get into contests, or ragchew for hours
with relatives or friends, you need to beef up.
Use your junkbox to tune up the network at 25 watts so you know what the
values are, and then go get something serious that will take what your
model says is needed for 5 kW, something with dissipation, etc, way in
excess of "ideal conditions" predicted by a network model. Do **not** just
barely cover 1.5 kW in your calculating.
Caps in tuning networks can wind up carrying many times the current
specified in models, as you tune away from "center" frequencies, or
experience wandering environment, like Z moving with wind, or rain
saturated ground, or trying to move up the band by switching a tuner in the
shack.
If you do QRO, get a vacuum cap or create the value with three or four
parallel ceramic doorknobs of the HEC HT50 variety for values of 500 pF or
less. The Russian flat doorknobs are probably the best for 3300 or 2200 pf
specifications. Most of the pictures have kBap (kVA) numbers on them.
If you can't locate **manufacturer** current ratings or Russian kBap
numbers, then don't use or don't buy. Invest in caps you KNOW are rated.
A contest will find you out if you go cheep, and heat often is run-away,
where increasing heat increases resistance. And then you're toast, because
the heat has quite possibly changed the value and characteristics of the
cap.
Prices have been going up on caps, and you may easily spend $100 or more to
get fixed caps that will do the job without heating up.
Don't ask me how I know this. :>) But I won't ever go cheep on tuner caps
again.
73, Guy K2AV
On Mon, Mar 13, 2017 at 9:42 AM Steve London <n2icarrl@gmail.com> wrote:
The characteristics of the doorknob are unknown. It's a junk box
special, with all of the lettering faded. Found an interesting article
on the web by I0IJ on RF vs. HF capacitors. I'll have to try
experimenting with more caps from the junk box.
Thanks for the suggestions.
73,
Steve, N2IC
On 03/12/2017 11:01 PM, Jim Kennedy wrote:
Garys suggestion is right on. I assume from the the cap you describe
that its a doorknob type. If so be sure it is RF rated and not a HV type
used in power supplies, Total different characteristics.
Jim
W7ouu
----- Original Message -----
From: "Gary Schafer" <garyschafer@largeriver.net>
To: n2ic@arrl.net, "Topband" <topband@contesting.com>,
towertalk@contesting.com
Sent: Monday, March 13, 2017 4:00:42 AM
Subject: Re: [TowerTalk] What's heating up ?
Probably the capacitor. Try 2 or 3 smaller values in parallel to make
your
2200pf. Then see if the drift is the same.
Or the coil wire size is too small.
73
Gary K4FMX
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Steve London
Sent: Sunday, March 12, 2017 10:42 PM
To: Topband; towertalk@contesting.com
Subject: [TowerTalk] What's heating up ?
I am experimenting with the N4KG reverse feed method for 160 meters on a
tower with a lot of stuff already on it. I like the possibility that
elevated radials will be an improvement over my terrible, dry,
mountaintop ground characteristics.
As I transmit, the SWR creeps up, pretty significantly. Something is
clearly getting warm.
Possibilities:
- I am matching the 160 feedline with an L-network (series L, shunt C).
TLW says 7 amps through the capacitor, with 3 watts of dissipation. The
capacitor measures 2200 pf, the usual transmitting capacitor,
cylindrical, about 3/4" in diameter.
- The M2 balun feeding the 40 meter beam on the same tower. I monitored
the SWR on the 40 meter beam while transmitting on 160. No change in 40
meter SWR.
- The M2 balun feeding the KT-36XA on the same tower. I monitored the
SWR on the KT-36XA while transmitting on 160. No change in KT-36XA meter
SWR.
- Gamma match on homebrew 15 meter beam. Gamma match is just a piece of
RG-8 dielectric/center conductor inside aluminum tubing.
Suggestions on how to troubleshoot this ? Start by disconnecting the
feedlines to the baluns ? And once I determine which component is
heating, what then ?
73,
Steve, N2IC
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