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Author Topic: Static bleed for end-fed  (Read 1581 times)
KF7QGA
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Posts: 19




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« on: November 11, 2012, 09:20:19 AM »

I've got an end-fed antenna which keeps building up enough static to blow my rig's pin diodes, which can only stand about 50V.  I need a DC path to ground, but a resistor doesn't seem to be the thing here: I've tried a 5W, 150K resistor, but it gets pretty warm.

I read on the internet (so it must be true, right?) that the near end of an end-fed antenna has very high voltages (hundreds or even thousands of volts).  If that's true, that would seem to explain why the bleeder resistor dissipates so much power.

I've got the idea that I could provide a better DC path to ground for my end-fed antenna using a choke, but I don't know if that's true, or what kind of choke to make or buy.  Can you please help?

Output power: 100W
Bands: 80M to 10M

73, Wayne KF7QGA
« Last Edit: November 11, 2012, 09:26:51 AM by KF7QGA » Logged
N3OX
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Posts: 8847


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« Reply #1 on: November 11, 2012, 09:35:02 AM »

I've tried a 5W, 150K resistor, but it gets pretty warm.

Warm like "burns up" or warm like "you notice it?"

All resistors run at their power rating or even half or a third of it will probably get warm.  5W resistors will often get damn hot when dissipating 5W   If your warm resistor means you're losing a couple watts out of a hundred, who cares?

The impedance at the end of an end fed half wave is high, but like 1000-5000 ohms, so hundreds of thousands shouldn't bleed off much power.  If the resistor is surviving, especially for long transmissions, don't worry about it.  A choke is probably out of the question.  Getting 150k impedance out of a choke is hard and probably requires a tuned choke.

You could switch to a link-coupled parallel-tuned matching unit, which I guarantee would fix your static problem with a low impedance DC ground through the large coil and galvanic isolation between link and main coil, but that's more work. Curious what you're using to match it that doesn't provide a DC ground?
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KF7QGA
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Posts: 19




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« Reply #2 on: November 11, 2012, 10:03:04 AM »

10 seconds of key-down gets the resistor hot enough to burn skin.

The rig is an Elecraft K2-100 with the KAT2 auto-tuner.  Between the tuner and the antenna I've got a balun, switchable between 4:1 and 1:1.  One of the balun's screw terminal is attached to my antenna; the other to ground.

There's a schematic of the balun here: http://www.elecraft.com/manual/BL2_Balun_Rev_B.pdf

If I understand the schematic right, the balun provides a DC path to ground when the balun's switch is in the 4:1 position, but not in the 1:1 position.  Which position the switch is in depends upon what band I'm on (some bands tune better with it in 4:1, others tune better with it in 1:1).

73, Wayne KF7QGA
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WB6BYU
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Posts: 13287




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« Reply #3 on: November 11, 2012, 10:14:46 AM »





Quote from: Wayne Conrad

10 seconds of key-down gets the resistor hot enough to burn skin.



Then you have something wrong:  did you check that the resistor really is 150K ohms
and not 150 ohms?



The voltage at the feedpoint of an end-fed wire antenna depends on the output power
and the length of the wire in wavelenghts:  wires that are a multiple of a half wave can
have impedances of a few thousand ohms, while those that are an odd multiple of a
quarter wave are often less than 100 ohms.

If the wire is very short in terms of wavelength, especially less than 1/8 wavelength,
then you can also have high voltages at the feedpoint, which will have a low resistance
but a high reactance.

So the dissipation in the wire would depend on the wire length and the band of operation.


But 150K ohms should be more than adequate to keep dissipation low in the resistor. As
long as the resistor doesn't get so hot that it is uncomfortable to touch, I wouldn't worry
about it.

You can add a RF choke in series with the resistor, which will increase the impedance, but
if the resistor really is 150K ohms then it won't make much difference.


Let's say you are using a half wave end-fed wire and the feedpoint impedance is 5000 ohms
(which is probably on the high side).  The RMS voltage is given by the square root of P * R,
so at 100 watts we have SQRT( 100 * 5000 ) = 707V, or about 1000V peak.  We'd use the
RMS voltage to determine the power dissipation in a 150K resistor at this voltage, which
would be 1/3 of a watt.  That's probably enough to take the chill off of a resistor, but
shouldn't get it too hot to touch.

With other wire lengths the dissipated power will be less, unless the wire is very short.
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KF7QGA
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Posts: 19




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« Reply #4 on: November 11, 2012, 10:55:02 AM »

Where's the icon for "red-faced?"

The resistor is marked as "1.5R5"; I took the 5 to be a multiplier and thought it was 150K.

I didn't see the "%" after the "5" until I looked more closely.

My Ohmmeter says it's about 3.5 ohms now.  It probably was much closer to 1.5 Ohms before I cooked it.
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WB6BYU
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Posts: 13287




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« Reply #5 on: November 11, 2012, 12:33:34 PM »

Don't feel bad - that's how many of us learned things!


Something like 100K 10W wire wound should be more than adequate, and
47K 5 watt should work in most cases.   A pair of them connected from
each side of the balun to the ground terminal would help to maintain
balance, though the resistance should be high enough that it isn't an
issue.  That would also ensure that the incoming line is grounded regardless
of the position of the switch.

Such power resistors are fairly common, though the higher resistances were
used more in tube equipment than in transistor gear.

Doesn't have to be non-inductive.
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K0ZN
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Posts: 1548




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« Reply #6 on: November 11, 2012, 01:40:23 PM »


Glad you found the problem, that is the good thing.

150K should do fine.  I use a string of 2 watt resistors totaling about 220K connected to each conductor of my ladderline right at the tuner output terminals, going
to ground to drain charges off my Center Fed Zepp.  Run legal limit and never have had a problem of any kind with the resistors. This is just for static discharge.
Obviously, that would not work for lightning protection. When lightning is around, I disconnect from the tuner and ground the line out doors. A little bit of a
pain but I REALLY don't want ladderline from an antenna terminated to ground INSIDE my house. To me the protection is more important than the minor inconvenience.
..... "Your results may vary...."

I use a homebrew choke of # 14 solid THHN house wire, close wound on about a 12" piece of 1 1/4 O.D. PVC pipe that shunts the base of my vertical to
ground. I can't remember how many turns it has.....maybe about 30 or so. I just kept adding turns until there was no detectable change in the SWR readings
on 40 M when the choke was hooked up or disconnected. It has been there a number of years without problem and of course that puts the vertical a DC
ground with almost no resistance. The base voltage on a 1/4 wave vertical is very low, not much of a concern, where as your end fed could have VERY high
voltage at the feed point, depending upon the band. In your case a combination of a moderate value resistor and an RF choke might make some sense if you
want to get perfectionist about it.

73,  K0ZN
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W9GB
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Posts: 2623




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« Reply #7 on: November 11, 2012, 08:14:31 PM »

Commercial AM verticals (> 200' in height) use Spark Gaps ...
 They get noisy as thunderstorms and cold fronts approach a transmitter site.

Sounds like you addressed issue.  Cushcraft R-series end-fed half-wave verticals use RFC.
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AD5X
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Posts: 1430




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« Reply #8 on: November 12, 2012, 05:14:19 AM »

...Let's say you are using a half wave end-fed wire and the feedpoint impedance is 5000 ohms
(which is probably on the high side).  The RMS voltage is given by the square root of P * R,
so at 100 watts we have SQRT( 100 * 5000 ) = 707V, or about 1000V peak.  We'd use the
RMS voltage to determine the power dissipation in a 150K resistor at this voltage, which
would be 1/3 of a watt....  

Hmmm...  Is this right?  In your example, 707V rms is right.  So power dissipated in a 150K resistor is 707^2/150K = 500,000/150,000 = 3.33 watts. 

I've been using 1M 1W metal oxide resistors (Mouser 71-RNX0381M00FKRB at about $4 ea) for static bleeds on high impedance antennas.  They are rated at 1.5KV and look very good up to 2 meters.

Phil - AD5X

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WB6BYU
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Posts: 13287




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« Reply #9 on: November 12, 2012, 06:40:57 AM »

Quote from: AD5X

Hmmm...  Is this right?  In your example, 707V rms is right.  So power dissipated in a 150K resistor is 707^2/150K = 500,000/150,000 = 3.33 watts. 



You are right - I appear to have fumble-fingered a decimal point somewhere.

Most end-fed wires will be rather less than 5000 ohms if you are matching them
with an auto-tuner, so this analysis really is looking at the worst case condition,
but that does say that a 47K resistor should have at least 10 watt rating.

One point to keep in mind is that most common 1/4 watt resistors are only rated for 250V,
so if you are using those then a string of, say, 100K resistors would be better than a single
1M resistor.  With power resistors this usually isn't a problem.

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