Rating for HF trap capacitor

R.L. "Bo" Thompson [KB5CNG]

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Aug 27, 1997, 3:00:00 AM8/27/97

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I would like to build a trap dipole for the HF bands and
would like to know the voltage rating on the capacitors I
will need to use. I plan on running about 100 Watts.
Thanks es 73
KB5CNG [Bo]
<thom...@ionet.net>

JohnsonHE

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Aug 28, 1997, 3:00:00 AM8/28/97

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>
>I would like to build a trap dipole for the HF bands and
>would like to know the voltage rating on the capacitors I
>will need to use. I plan on running about 100 Watts.

All you can get. The voltage is high enough in the first place, but the
environment they have to live in is horrendous. Centralab 858-S series
(rated 5KV) work for me at the KW level.

Assume an end impedance of maybe 2500 ohms, the voltage there from 100
watts is 500 volts. I guarantee a 500 volt capacitor will fail at exactly
the time the new country comes on that you absolutely have to have in the
logbook.

W4ZCB

Tom Bruhns

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Aug 28, 1997, 3:00:00 AM8/28/97

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Art Jackson (artj...@mindspring.com) wrote, in reply to Bo's
question below:

: Most home built traps are designed around the TV
: High-voltage capacitors
: with voltage ratings of 15KV or higher...If you can get a
: copy of an ARRL Antenna Book, there is a section devoted to
: Trap dipoles and construction of traps for them...Good luck

: R.L. "Bo" Thompson [KB5CNG] wrote:
:
: > I would like to build a trap dipole for the HF bands and

: > would like to know the voltage rating on the capacitors I
: > will need to use. I plan on running about 100 Watts.

I'm curious about this. It seems to me that 15kV rating is grossly more
than needed for 100-watt level traps, though it might help for nearby
lightening strikes. My rule-of-thumb would be that the voltage at the
end of the wire would be less than sqrt(20kohms*100W), or around 2000
volts peak. That's from expected antenna impedance, for quite small
wire diameter. An alternative way to look at it would be to assume,
say, 10 watts dissipation in the trap (which seems like quite a lot for
a good design!) and use that and the coil reactance and Q to get the
voltage. For example, if Q = 300 and XL = XC = 1000 ohms at resonance,
the trap would look like 300k ohms at resonance, and 10 watts
dissipation would be about 2500 volts peak. These seem like
conservative numbers in both cases to me, and they are pretty similar.
But what I'm looking for is a sanity check from those who've
experimented with trap antennas and may have found that you need to use
much higher voltage parts than these. (I'm probably talking more from
the engineer part of my brain here, which wants to design reliable
things, but not waste money where it's not needed...the ham part of
my brain says it may not be worth wasting effort thinking about; just
use high voltage parts and not worry about it.)

--
Cheers,
Tom
to...@lsid.hp.com

Art Jackson

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Aug 28, 1997, 3:00:00 AM8/28/97

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Tom Bruhns wrote:
>
> Art Jackson (artj...@mindspring.com) wrote, in reply to Bo's
> question below:
>
> : Most home built traps are designed around the TV
> : High-voltage capacitors
> : with voltage ratings of 15KV or higher...If you can get a
> : copy of an ARRL Antenna Book, there is a section devoted to
> : Trap dipoles and construction of traps for them...Good luck
>
> : R.L. "Bo" Thompson [KB5CNG] wrote:
> :
> : > I would like to build a trap dipole for the HF bands and
> : > would like to know the voltage rating on the capacitors I
> : > will need to use. I plan on running about 100 Watts.
>
> I'm curious about this. It seems to me that 15kV rating is grossly more
> than needed for 100-watt level traps, though it might help for nearby
> lightening strikes. My rule-of-thumb would be that the voltage at the
> end of the wire would be less than sqrt(20kohms*100W), or around 2000
> volts peak.

At the 100 watt level, much lower voltage ratings would
suffice...5KV would be OK for that level but it doesn't leave much
overhead... If you decide to add an amp to increase power up to KW
levels then you would have to take the antenna down and rebuild
the traps...In the early days
of trap building the TV High Voltage capacitors were readily
available
at a reasonable price and they were rated at 15KV and up...I
didn't mean to imply that Bo's 100 watt traps required 15KV, only
that with that rating, he could rest easy and not have to worry
about burning them up.
The slight (if any) increase in cost would be worth it to provide
the
extra safety margin for RF voltages and also induced voltage
spikes from nearby lightning strikes...I go along with the Ham
side of your brain...
73

> But what I'm looking for is a sanity check from those who've
> experimented with trap antennas and may have found that you need to use
> much higher voltage parts than these. (I'm probably talking more from
> the engineer part of my brain here, which wants to design reliable
> things, but not waste money where it's not needed...the ham part of
> my brain says it may not be worth wasting effort thinking about; just
> use high voltage parts and not worry about it.)
>
> --
> Cheers,
> Tom
> to...@lsid.hp.com

Linley F Gumm

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Aug 28, 1997, 3:00:00 AM8/28/97

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R.L. "Bo" Thompson [KB5CNG] wrote:
>
> I would like to build a trap dipole for the HF bands and
> would like to know the voltage rating on the capacitors I
> will need to use. I plan on running about 100 Watts.

> Thanks es 73
> KB5CNG [Bo]
> <thom...@ionet.net>

A handy way to predict what voltages will appear across a trap
is to model the antenna, including the trap, with a NEC-type
of antenna CAD software like EZNEC. It will compute the
voltage for you.

The results are suprisingly modest (low numbers of kV) unless you are
running high power.

Linley Gumm
K7HFD
I don't speak for my company.

Roy Lewallen

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Sep 1, 1997, 3:00:00 AM9/1/97

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In article <34078D...@interaccess.com>,

Robert Mansfield <bman...@interaccess.com> wrote:
>R.L. "Bo" Thompson [KB5CNG] wrote:
>>
>> I would like to build a trap dipole for the HF bands and
>> would like to know the voltage rating on the capacitors I
>> will need to use. I plan on running about 100 Watts.
>> Thanks es 73
>> KB5CNG [Bo]
>> <thom...@ionet.net>
>

>The easy way to do this is to model the antenna with a program like
>EZNEC and
>put a hi value resistor( 10^10) in place of the trap and then look at
>the voltage
>in the load data.

No need to replace the trap with a resistor. EZNEC will tell you the actual
voltage across the capacitor in the Load Data info. Don't forget to set the
power level in the Options Menu to the amount of power actually being
supplied.

Roy Lewallen, W7EL

n8...@ix.netcom.com

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Sep 1, 1997, 3:00:00 AM9/1/97

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Folks,
At the risk of being flamed, you might give some usefull data to some
who might not have all day to play with the computer or forbid not have
EZNEC. Like.....for most traps at the 100w level, caps in the range of
5KV work just fine, as long as they are kept dry (out of the rain).

Greg N8KFE

JohnsonHE

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Sep 2, 1997, 3:00:00 AM9/2/97

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>
>Folks,
>At the risk of being flamed, you might give some usefull data to some who
might not have all day to play with the computer or forbid not have EZNEC.
Like.....for most traps at the 100w level, caps in the range of 5KV work
just fine, as long as they are kept dry (out of the rain).
>
>Greg N8KFE

Bet I've built a dozen trapped antennas' in the last 50 years around the
5 KV rated Centralab 858-S. I've never blown ONE at the KW level. Even a
little above a KW from time to time.

W4ZCB

R.L. "Bo" Thompson [KB5CNG]

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Sep 2, 1997, 3:00:00 AM9/2/97

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Thanks to all who replied, both via the NG and E-Mail. I recieved
much valuble input that will help me to design and build my antenna.
73 to all and thanks again.
Bo KB5CNG
<thom...@ionet.net>