At my dipole's feedpoint I have a coil of rg8 mini coax
wrapped 6 turns in a 2-5/8" dia circle. I found a program
for calculating coaxial traps for dipoles and here are the
numbers it gave me:

Form diameter 2.375"
Coax diameter 0.242"
Capacitance/ft 25.20pf
Turns 6.000
Coil length 1.45"
Coax length 50.30"
End sensitivity 99.130 khz/inch
Turn sensitivity 257.94 khz/inch
Length/Diameter 0.550
Inductance (L) 2.342 uH
Capacitance (C) 105.620 pF
Reactance (X) 148.90 ohms


The program says this coil is resonant (?) at 10.120 mhz

Now here comes the really dumb question - will this cause
a problem on 30 meters? I seem to hear OK but I've not
yet tried to make any contacts...

My SWR meter indicates a 3:1 SWR there but my nifty
Z11 autotuner tunes it just fine.......

What am I not understanding here?

Ken KG0WX

Ken:

You haven't constructed a trap. A trap is a parallel resonant circuit,
meaning the inductance and capacitance are in parallel with each other. At
resonance it presents a high impedance. A trap can be constructed with
coaxial cable, but to get the inductance and capacitance in parallel you
have to cross connect the inner and outer conductors. See:
http://www3.sympatico.ca/wrgraham/ve...coaxtraps.html

You have wound the coax into a choke inductor but the capacitance you
calculated is between the inner and outer conductors and is not in parallel
with the inductance of the windings.

If your calculation of the inductance is correct (2.342 uh), by itself it
doesn't have a very high impedance at 10 mhz and doesn't serve very well as
a choke. It may be doing more harm than good since a resonant dipole should
have better than a 3:1 SWR. Have you tried the dipole without it?

Roger K6XQ

"Roger Leone" wrote in message
...
Ken:

You haven't constructed a trap. A trap is a parallel resonant circuit,
meaning the inductance and capacitance are in parallel with each other.
At
resonance it presents a high impedance. A trap can be constructed with
coaxial cable, but to get the inductance and capacitance in parallel you
have to cross connect the inner and outer conductors. See:
http://www3.sympatico.ca/wrgraham/ve...coaxtraps.html

You have wound the coax into a choke inductor but the capacitance you
calculated is between the inner and outer conductors and is not in
parallel
with the inductance of the windings.

If your calculation of the inductance is correct (2.342 uh), by itself it
doesn't have a very high impedance at 10 mhz and doesn't serve very well
as
a choke. It may be doing more harm than good since a resonant dipole
should
have better than a 3:1 SWR. Have you tried the dipole without it?

Roger K6XQ




The dipole resonates (or at least has a 1:1 match) at 2.31 mhz. Without
the coil, I had problems with RF in the shack. A nice fellow changed my
mind from buying a balun to simply winding the coil at the feedpoint.
It seems to have worked. I suppose I could wind more turns to get a
higher inductance - what amount would you advise?

Thanks for your insight as to the differance between my coil and a trap!

Ya learn something new everyday..... :-)

72's de Ken KG0WX

On Sun, 24 Aug 2003 08:38:50 -0600, "Ken Bessler"
wrote:

Inductance (L) 2.342 uH
Capacitance (C) 105.620 pF
Reactance (X) 148.90 ohms


The program says this coil is resonant (?) at 10.120 mhz

Now here comes the really dumb question - will this cause
a problem on 30 meters? I seem to hear OK but I've not
yet tried to make any contacts...

My SWR meter indicates a 3:1 SWR there but my nifty
Z11 autotuner tunes it just fine.......

What am I not understanding here?

Ken KG0WX

Hi Ken,

Not much reactance overall for one. I've lost track what bands you
want to work, but the SWR would suggest not much Q either. There is
also the matter of how long your cable is too at any particular
wavelength; at some lengths, like half wave, it just doesn't matter if
you had a choke or not (the radiation resistance would snub the
current anyway).

You got 3:1 for any number of reasons, but your tuner took care of it.
That's what a tuner is for. [However, as a solution it is unrelated
to your original problem of a hot coax shield.]

Feeling a little chagrined at accelerating the super glue bond? ;-)

Offhand I would offer that you are probably out of the woods. Just
leave it alone barring repeated symptoms that lead you here.

73's
Richard Clark, KB7QHC

A coil of coax has capacitance between turns. This makes the coil look
like a parallel resonant circuit -- like a trap -- at some frequency.
The most effective choke is self resonant at the frequency of use. The
diameters and numbers of turns for coax choke baluns given in the _ARRL
Antenna Book_ are ones that result in self-resonance at the center of
the recommended operating range. These were determined by measurement
with a vector impedance meter. An antenna analyzer can be used just as
well for measurement of choke impedance, by tying the inner and outer
conductors together, connecting one end of the choke to the analyzer
"hot" lead and the other to the analyzer ground. The higher the
impedance, the better.

Roy Lewallen, W7EL

Roger Leone wrote:
Ken:

You haven't constructed a trap. A trap is a parallel resonant circuit,
meaning the inductance and capacitance are in parallel with each other. At
resonance it presents a high impedance. A trap can be constructed with
coaxial cable, but to get the inductance and capacitance in parallel you
have to cross connect the inner and outer conductors. See:
http://www3.sympatico.ca/wrgraham/ve...coaxtraps.html

You have wound the coax into a choke inductor but the capacitance you
calculated is between the inner and outer conductors and is not in parallel
with the inductance of the windings.

If your calculation of the inductance is correct (2.342 uh), by itself it
doesn't have a very high impedance at 10 mhz and doesn't serve very well as
a choke. It may be doing more harm than good since a resonant dipole should
have better than a 3:1 SWR. Have you tried the dipole without it?

Roger K6XQ

Winding the coil at the feedpoint creates a choke, known also as a
"current balun". It serves the primary function of a balun which, for
coax, is to prevent current flow on the outside of the coax.

As I mentioned in another posting, the most effective choke is one
that's self-resonant at the frequency of operation. A typical choke make
by winding the wire in a flat coil like a coil of rope is very effective
over about three ham bands, if it's resonant at the center of the range.

Increasing the number of turns could make your choke more of less
effective, depending on where its self resonance now is compared to your
operating frequency.

Roy Lewallen, W7EL

Ken Bessler wrote:

The dipole resonates (or at least has a 1:1 match) at 2.31 mhz. Without
the coil, I had problems with RF in the shack. A nice fellow changed my
mind from buying a balun to simply winding the coil at the feedpoint.
It seems to have worked. I suppose I could wind more turns to get a
higher inductance - what amount would you advise?

Thanks for your insight as to the differance between my coil and a trap!

Ya learn something new everyday..... :-)

72's de Ken KG0WX

"Richard Clark" wrote in message
...
Hi Ken,

Not much reactance overall for one. I've lost track what bands you
want to work, but the SWR would suggest not much Q either. There is
also the matter of how long your cable is too at any particular
wavelength; at some lengths, like half wave, it just doesn't matter if
you had a choke or not (the radiation resistance would snub the
current anyway).

You got 3:1 for any number of reasons, but your tuner took care of it.
That's what a tuner is for. [However, as a solution it is unrelated
to your original problem of a hot coax shield.]

Feeling a little chagrined at accelerating the super glue bond? ;-)

Offhand I would offer that you are probably out of the woods. Just
leave it alone barring repeated symptoms that lead you here.

73's
Richard Clark, KB7QHC

I have used baking soda as an accelerator for years - works great
everytime. Just don't let the puff of smoke that comes out get into
your nostril membranes or your eyes..... nasty reaction, then!

Winding the coil had several good effects:

1) No more rfi in my PC speakers
2) No more fluctuating SWR readings
3) More consistant (and faster) autotuner action

So, it works - thanks to all the advice I got here (it really
makes me feel like the tech + KB0PWC I started out as),
I've got a viable radiation system.

I'll have to wait for the winter 160m season to see how
well the system gets out on that band - W7EL Roy suggested
I model the antenna on EZNEC and compare the resonance
points and SWR bandwidth. I think (?) I have the program
figgured out and it appears his worry about the bricks in my
building effecting the antenna are somewhat proper.

However, the performance of this antenna in the AM broad-
cast band is SUPERB! I'm hearing stations all over the band
at night, from as far away as Chicago and Los Angeles. So
I've got hope the antenna won't be a complete flop on 160.

Last night I worked several of the Ohio QSO party stations
on 40 meters with many amazed at my "QRP with a wire"
setup...... not to bad from Colorado.....

72's de Ken KG0WX

On Sun, 24 Aug 2003 10:52:40 -0600, "Ken Bessler"
wrote:


However, the performance of this antenna in the AM broad-
cast band is SUPERB! I'm hearing stations all over the band
at night, from as far away as Chicago and Los Angeles. So
I've got hope the antenna won't be a complete flop on 160.

Last night I worked several of the Ohio QSO party stations
on 40 meters with many amazed at my "QRP with a wire"
setup...... not to bad from Colorado.....

72's de Ken KG0WX

Hi Ken,

Not to rain on your parade, but as a teenager in Colorado Springs,
back in the 60's, I had a 50' longwire traced along the fence rail at
roughly 6'. I could AM DX both coasts on my RBB-1 (WWII shipboard
receiver which weighed as much as a pocket battleship).

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sun, 24 Aug 2003 10:52:40 -0600, "Ken Bessler"
wrote:


However, the performance of this antenna in the AM broad-
cast band is SUPERB! I'm hearing stations all over the band
at night, from as far away as Chicago and Los Angeles. So
I've got hope the antenna won't be a complete flop on 160.

Last night I worked several of the Ohio QSO party stations
on 40 meters with many amazed at my "QRP with a wire"
setup...... not to bad from Colorado.....

72's de Ken KG0WX

Hi Ken,

Not to rain on your parade, but as a teenager in Colorado Springs,
back in the 60's, I had a 50' longwire traced along the fence rail at
roughly 6'. I could AM DX both coasts on my RBB-1 (WWII shipboard
receiver which weighed as much as a pocket battleship).

73's
Richard Clark, KB7QHC

That just reassures me I'm on the right track. I'd have picked
up more stations but they were all on top of each other in a big
jumble...

Ken

On Sun, 24 Aug 2003 17:01:40 GMT, Richard Clark hath writ:
On Sun, 24 Aug 2003 10:52:40 -0600, "Ken Bessler"
wrote:

However, the performance of this antenna in the AM broad-
cast band is SUPERB! I'm hearing stations all over the band
at night, from as far away as Chicago and Los Angeles. So
I've got hope the antenna won't be a complete flop on 160.

Last night I worked several of the Ohio QSO party stations
on 40 meters with many amazed at my "QRP with a wire"
setup...... not to bad from Colorado.....

72's de Ken KG0WX

Not to rain on your parade, but as a teenager in Colorado Springs,
back in the 60's, I had a 50' longwire traced along the fence rail at
roughly 6'. I could AM DX both coasts on my RBB-1 (WWII shipboard
receiver which weighed as much as a pocket battleship).

73's Richard Clark, KB7QHC

Not to rain on _your_ parade g, but, back then, licensed radio
transmitters were about the only "radiators" you had to contend with.
(Well, maybe not in the 60's -- but, I remember the 50's....)
Oh for the days when you could tune to an unoccupied frequency and
turn both the AF and RF control up to max and listen to the cosmic
background.

73 Jonesy
--
| Marvin L Jones | jonz | W3DHJ | OS/2
| Gunnison, Colorado | @ | Jonesy | linux __
| 7,703' -- 2,345m | config.com | DM68mn SK