Topband: [TowerTalk] What's heating up ?

Mon Mar 13 13:13:26 EDT 2017

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 at 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 at largeriver.net>
>>> To: n2ic at arrl.net, "Topband" <topband at contesting.com>,
>> towertalk at 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 at contesting.com] On Behalf Of
>>>> Steve London
>>>> Sent: Sunday, March 12, 2017 10:42 PM
>>>> To: Topband; towertalk at 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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