[Elecraft] OT antenna question

Wed Jan 13 02:41:45 EST 2016

Vic:
Sounds like you have some things to check. Joe and others have given good
advice. However I question your overall design. It sounds like you have a
number of mismatches in your system. For instance is the feed point
impedance of your 20M dipole >600 ohms at 40 meters?  I think not but that
would be your match for "true ladder line". Then you switch to 450 ohm
ladder line at the static box. Have you checked for fried spiders across the
spark gaps? Then you go through a couple of caps, value unknown into a 4:1
balun which would bring you down about a 100 ohms (neglecting the caps) at
the coax. Seems like you would be adding series inductance not capacitance
and then maybe a 6 or 8 to 1 balun. Overall it seems like you are tuning
your feedline not your antenna.   Also you do not say how much power you are
running but I am betting you are getting less than 10% to your antenna. If
you are running 500 watts that would leave 450 watts dissipated in your
feedline. Ouch! It could be your coax is melting. 
Finally it sounds like you have multiple reflection points in your system
which isn't going to make your SWR meters very accurate. I recommend you try
simulating your transmission line in SPICE or some other simulation CAD
program and see what you really have. 
73
Fred, AE6QL

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Joe
Subich, W4TV
Sent: Tuesday, January 12, 2016 8:59 PM
To: Elecraft Reflector
Cc: Vic Rosenthal 4X6GP/K2VCO
Subject: Re: [Elecraft] OT antenna question

On 1/12/2016 10:57 PM, Bob McGraw K4TAX wrote:
 > If your tuner uses disk ceramic caps, as many do, these can be  > heating
thus not being able to handle the RF current.  They heat  > and cool and
change value which in turn changes tuning.

Particularly if you are using a 4:1 balun!  On 40 meters the impedance of
the half size antenna is *very low*.  The 4:1 balun will further decrease
the impedance seen by the tuner causing it to work at its absolute worst
efficiency (highest circulating current).

At the very least see if you can extend the dipole to 44', then get some
Smith Chart software and see if you can find a compromise feedline length
that provides a reasonable impedance on all bands, and finally replace the
4:1 balun with a high quality 1:1 current balun.

By extending the dipole you should be able to find a compromise feedline
length that will provide a 100 - 300 Ohm impedance on all bands - something
the tuner will be much happier to with and be far more efficient than
working at best than 10 Ohms as it probably is doing on 40 meters.

73,

   ... Joe, W4TV

On 1/12/2016 10:57 PM, Bob McGraw K4TAX wrote:
> If your tuner uses disk ceramic caps, as many do, these can be heating
thus not being able to handle the RF current.  They heat and cool and change
value which in turn changes tuning.
>
> Replacing them with suitable RF current rated units is the solution.
>
> Bob, K4TAX
>
>
> Sent from my iPhone
>
>> On Jan 12, 2016, at 5:27 PM, Guy Olinger K2AV <k2av.guy at gmail.com> wrote:
>>
>> Hi Vic,
>>
>> Your story does suggest trouble at very high current points. Just a 
>> list of things below I've heard or seen to stir up a new idea or two. 
>> No opinion on which if any makes any sense in your situation.
>>
>> ---------
>>
>> Insect nests in tubular spacers on open wire.
>>
>> Spacers have carbon tracks.
>>
>> Material inside the balun housing is much hotter than touchable material.
>>
>> Stranded wire in open wire has been waterlogged, corroded and the 
>> remaining conductor material at a current max along the line is heating
up.
>>
>> Wire inside insulation is nearly all broken, or is broken, and 
>> contact is miscellaneous and highly resistive.
>>
>> Insulation on the wire is significantly compromised by ultra violet 
>> or critter nibbling, letting in water to the stranded conductor. 
>> Advice, has been to use bare solid #12 copper or larger for open wire 
>> that is carrying large standing waves, to handle the current maximums.
>>
>> Check heat all around the entire core. I have burned up some number 
>> of cores before I understood the materials and engineering. On one 
>> all the damage was on a spot that comprised only 15 degrees of the
circumference.
>>
>> Electrical connections in aluminum elements made of telescoping 
>> tubing go highly resistive as water is boiled out of the joints.
>>
>> Connections made of dissimilar metals/materials go bad and become 
>> more resistive as water is boiled out of the joint.
>>
>> Tuner rotary switches going bad.
>>
>> The rolling contact on on a variable coil losing its tension 
>> (numerous ways for this to happen depending on construction), contact 
>> is very small and becomes worse as it heats up. Fixed by repairing 
>> mechanism for maintaining contact.
>>
>> Parted conductors at conductor joints due to metal erosion at the 
>> contact point.
>>
>> --------
>>
>> Your troubles would have me taking down the dipole, rebuilding it 
>> with all mating surfaces wire-brushed and reassembled with clear 
>> silicone dielectric grease, completely new hardware likewise treated, 
>> with all balanced conductors replaced new, using bare solid #12 
>> outdoors, and brand new runs indoors. Old wire to recycling.
>>
>> Hope this has kicked off a new idea for you.  73 and good luck.  Guy 
>> K2AV
>>
>> On Tue, Jan 12, 2016 at 4:24 PM, Vic Rosenthal 4X6GP/K2VCO < 
>> k2vco.vic at gmail.com> wrote:
>>
>>> I'm having a problem which has me stumped. I'm going to describe my 
>>> complete antenna and feed system because something in it is 
>>> misbehaving and I don't know what!
>>>
>>> My system works on all bands from 40 to 10 (or it should).
>>>
>>> The antenna is a full-size 20m rotary dipole. It is all aluminum 
>>> tubing, no traps or stubs. Just a dipole. I am feeding it with about 
>>> 30 feet of "true ladder line," which is open wire line made of #16 
>>> insulated wire spaced about 3-1/4" with black PVC spacers every 18" 
>>> or so, except near the antenna and the rotor where I've added extra 
>>> ones so that the spacing doesn't change when the antenna rotates.
>>>
>>> The line comes into the shack and is connected to a static drain, 
>>> which is a box with two 10-megohm high voltage resistors to ground 
>>> and a couple of spark gaps. Then a piece of 450-ohm window line 
>>> about 3 feet long connects it to a pair of large air variable 
>>> capacitors in series with each leg which knock out some of the 
>>> reactance on 40m to make it possible to tune more easily. Then a 
>>> very short piece of window line connects to a big 5kW DX Engineering 
>>> 4:1 balun, spec'ed for tuner service, and finally via a piece of RG-213
18" long, to a T-network tuner.
>>>
>>> My K3 drives a TL922 amp and I have an SWR meter in line.
>>>
>>> Now here is my problem: it works OK on all bands except 40 meters. 
>>> On 40, it tunes up fine with low power, but when I run more than a 
>>> couple of hundred watts, after perhaps 10 seconds of key-down, the 
>>> SWR starts to climb. I have watched it go to 4:1 before I stop 
>>> sending for fear of destroying something.
>>>
>>> The SWR rises both on the meter in the tuner and the extra one I 
>>> have in line.
>>>
>>> Classic symptoms of something heating up. But what?
>>>
>>> - The tuner components are all cold.
>>> - The coax to the balun and its connectors are cold.
>>> - The balun itself is just barely perceptibly warmer (I have to 
>>> touch the core to tell).
>>> - The window line, the static drain resistors, the air capacitors 
>>> and all the connections in the shack are cold.
>>>
>>> I know the SWR is astronomical on 40 meters, so currents and 
>>> voltages are high. But nothing in the shack seems to be heating up. 
>>> Any more ideas of where to look?
>>>
>>> --
>>> 73,
>>> Vic, 4X6GP/K2VCO
>>> Rehovot, Israel
>>> http://www.qsl.net/k2vco/
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