Common mode current balun
Steve Beyers
5.9-j285 ohms. Feedline 290 ohms, 4-wire, open, balanced. 1:1 current
balun, at antenna base, to minimize (eliminate?) common mode currents.
Question: What should the impedance of the balun be? Some multiple of
the antenna impedance? Of the feedlime impedance? Of the larger of the
two? Do these (or some other parameter) apply to all antenna
impedance/feedline impedance situations?
Steve W9HJW
W6RCecilA
>
> Situation: Vertical antenna, ground mounted, buried radials, impedance
> 5.9-j285 ohms. Feedline 290 ohms, 4-wire, open, balanced. 1:1 current
> balun, at antenna base, to minimize (eliminate?) common mode currents.
Since this is a single frequency/band vertical, I would install a matching
network at the base of the antenna and use 50 ohm coax. IMO, it's not
logical to use hi-ZØ feedline with lo-R loads.
Just because a load is 290 ohms at 89 degrees does NOT mean it is a good
match for 290 ohm feedline. It's not convenient for me to calculate the
SWR at the moment but it will be sky high.
--
73, Cecil, W6RCA http://people.delphi.com/CecilMoore
Don W6JL
I am not sure why you have elected to use a 4-wire transmission line, but
a common-mode choke should have a high impedance, at least 10X the impedance
of the load, to discourage common-mode currents from flowing, and allowing
the
differential (desired) currents to flow freely Your line will obviously
have high SWR,
which is no problem for low loss lines at HF.
From your feedpoint impedance numbers, it looks like you are driving
about
a 1/8 wave tall vertical. For single-band use, you can tune out the
reactance
at the bottom and convert the R to 50 ohms, and feed with coax. Or the
same
for multiband, with a remote-tuned matching network. Obviously, using
open wire and tuning at the shack end is fine, too.
73 and GL,
Don, W6JL
Steve Beyers wrote in message <369A32...@trianglenet.net>...
>Situation: Vertical antenna, ground mounted, buried radials, impedance
>5.9-j285 ohms. Feedline 290 ohms, 4-wire, open, balanced. 1:1 current
>balun, at antenna base, to minimize (eliminate?) common mode currents.
>
Roy Lewallen
of transmission line inserted in the system. If the differential
impedance equals the Z0 of your transmission line, the balun looks
like an extra length of line. If the balun's differential Z is
different, it'll look like a piece of line of that Z0 inserted in the
system. (In many cases, this doesn't make much difference.)
The balun common-mode impedance should be considerably higher than the
common-mode impedance to ground via the antenna and feedline. This
latter value is hard to estimate, but I'd guess that a conventional
balun with common-mode impedance of several hundred to one or two
thousand ohms should be fine for your antenna.
Roy Lewallen, W7EL
Thomas C Sefranek
W6RCecilA wrote:
> Steve Beyers wrote:
> >
> > Situation: Vertical antenna, ground mounted, buried radials, impedance
> > 5.9-j285 ohms. Feedline 290 ohms, 4-wire, open, balanced. 1:1 current
> > balun, at antenna base, to minimize (eliminate?) common mode currents.
>
> Since this is a single frequency/band vertical, I would install a matching
> network at the base of the antenna and use 50 ohm coax. IMO, it's not
> logical to use hi-ZØ feedline with lo-R loads.
Well, at least try to match the 5.9 ohms to 290 ohm line.
(I think matching to 50 ohms is easier also Cecil.)
> Just because a load is 290 ohms at 89 degrees does NOT mean it is a good
> match for 290 ohm feedline. It's not convenient for me to calculate the
> SWR at the moment but it will be sky high.
Is the feed line 300 ohm because it's 1,000 feet long?
>
> --
> 73, Cecil, W6RCA http://people.delphi.com/CecilMoore
--
Thomas C. Sefranek WA1RHP
ARRL Instructor, Technical Specialist, VE Contact.
http://hamradio.cmcorp.com/41_Repeater/repeater.html
http://hamradio.cmcorp.com/inventory/Inventory.html
George T. Baker
72/73, George
Amateur Radio W5YR, in the 53rd year and it just keeps getting better!
AutoPOWER Systems, Fairview, TX (30 mi NE Dallas) Collin County
QRP-L QRP-ARCI FISTS NORCAL ZOMBIE #522 ARS 10-X 33.2 N 96.6 W EM13RE
Walter Maxwell wrote:
> >
> An antenna terminal impedance of 5.9 -j285 ohms terminating a 290-ohm
> line yields an SWR of 96.6:1. Equation and HP calculator data in
> 'Reflections', Appendix 3 and 4, respectively.
>
> Walt Maxwell, W2DU
Walter Maxwell
Steve Beyers
>
> Steve OM,
>
> I am not sure why you have elected to use a 4-wire transmission line, but
> a common-mode choke should have a high impedance, at least 10X the impedance
> of the load, to discourage common-mode currents from flowing, and allowing
> the
> differential (desired) currents to flow freely Your line will obviously
> have high SWR,
> which is no problem for low loss lines at HF.
> From your feedpoint impedance numbers, it looks like you are driving
> about
> a 1/8 wave tall vertical. For single-band use, you can tune out the
> reactance
> at the bottom and convert the R to 50 ohms, and feed with coax. Or the
> same
> for multiband, with a remote-tuned matching network. Obviously, using
> open wire and tuning at the shack end is fine, too.
>
> 73 and GL,
> Don, W6JL
>
> Steve Beyers wrote in message <369A32...@trianglenet.net>...
> >5.9-j285 ohms. Feedline 290 ohms, 4-wire, open, balanced. 1:1 current
> >balun, at antenna base, to minimize (eliminate?) common mode currents.
> >
> >the antenna impedance? Of the feedlime impedance? Of the larger of the
> >two? Do these (or some other parameter) apply to all antenna
> >impedance/feedline impedance situations?
> >
> > Steve W9HJW
Don -
Thanks for a thoughtful and informed answer. Yes, about 1/8 wave.
I'm using a 32 foot tower on 40m now, and would like to be able to use
it on 80m also. I got the 4-wire idea from the handbook - it said it
would improve the line current balance, as well as cut the impedance in
half over 2-wire line. Works good.
With SWR=96 (from EZNEC) I calculated a total line loss of 1.9 db/100
ft. With a short line (86 ft.) this seems acceptable. At 100 watts:
Emax = 1668v
Epeak = 2360v
Emin = 17v
Imax = 5.8a
Imin = 0.06a
These are all well within the components I am using. The balun would be
at a high current-low voltage point (antenna base), so insulation/arcing
should not be a problem.
SWR is much lower on the higher-frequency bands, so if the balun works,
I should be able to tune all bands from 80m up from the shack (I use
homebrew link couplers). Everything is buried in the snow right now, so
it may be a while before I try it.
Thanks again for the answer. I have a tendency to mess up calculations,
so if you (or anyone) would want to check them, I'd sure appreciate
hearing the result.
Regards, Steve
W6RCecilA
> With SWR=96 (from EZNEC) I calculated a total line loss of 1.9 db/100
> ft. With a short line (86 ft.) this seems acceptable. At 100 watts:
Hi Steve, with an SWR of 96:1 and ZØ = 290 ohms, the impedance seen
at the transmitter will vary from 3 ohms to 27,840 ohms. How the
heck are you going to match those impedances? Where are you going
to get a 250K ohm balun? Try calculating the losses in your tuner
when matching 50 ohms to 3 ohms.
Steve Beyers
Hi, Cecil -
Matching in the shack has not been a problem - I've been doing it
for quite a while using a homebrew link coupler. The problem is with
the balun at the antenna base - it heats up when I run over 25 watts or
so, and the SWR starts to drift around. Hence my interest in the balun.
With a load (the antenna) impedance of 5.9 - j285 ohms complex, the
scalar impedance would be roughly 286 ohms. Ten times that would be
2860 ohms. My idea is to try an air-core balun. Just scramble-wind two
strands of insulated wire into a coil like they do with coax. Sixteen
bifilar turns in a 12-inch diameter coil should do it. At first I
thought of winding it on a conventional cylindrical form, but this would
be a lot easier. Roy said in another thread that doing it that way will
increase distributed capacitance, but maybe I'll get lucky and hit
self-resonance - he says that's good :-).
On calculating tuner losses, I don't know how to do that - could
you enlighten me? Nothing heats up in my tuner. Where would the losses
be?
If I can get this to work, according to EZNEC the antenna should
work almost as well as a full-size 80m quarter wave vertical. The
vertical take-off angles are 23 and 21 degrees respectively. The lower
radiation angles at the -3db points are both 7 degrees. The gains are
2.05 dbi and 1.93 dbi.
Like I've said before, I have a tendency to mess up calcualtions.
I'm too old to have any pride of authorship, so please don't hesitiate
to point out mistakes. Thanks for the reply.
Regards, Steve
W6RCecilA
> Matching in the shack has not been a problem - I've been doing it
> for quite a while using a homebrew link coupler.
Link coupling eliminates the need for a balun at the shack end.
> The problem is with
> the balun at the antenna base - it heats up when I run over 25 watts or
> so, and the SWR starts to drift around. Hence my interest in the balun.
A choke-type balun might work better. Examples are on my web page.
> On calculating tuner losses, I don't know how to do that - could
> you enlighten me? Nothing heats up in my tuner. Where would the losses
> be?
Assuming a particular load, the current through the coil and coil Q
can be estimated and losses calculated from that.
> If I can get this to work, according to EZNEC the antenna should
> work almost as well as a full-size 80m quarter wave vertical. The
> vertical take-off angles are 23 and 21 degrees respectively. The lower
> radiation angles at the -3db points are both 7 degrees. The gains are
> 2.05 dbi and 1.93 dbi.
IMO, it would be easier to match at the base but I sure don't want
to discourage experimentation.
--
73, Cecil, W6RCA http://people.delphi.com/CecilMoore/mystery.htm
Walter Maxwell
worth in occasionally. Incidentally, Amateur Radio W2DU is in its 66th
year, and like W5YR, it also just keeps getting better. Before I
forget to ask, did your call ever belong to a college or university?
In the 20s and 30s two-letter calls beginning with 'y' were routinely
assigned exclusively to colleges and universities.
73, Walt
On Mon, 11 Jan 1999 23:41:13 -0600, "George T. Baker"
<w5...@swbell.net> wrote:
>Welcome back, Walt!
>
>72/73, George
>Amateur Radio W5YR, in the 53rd year and it just keeps getting better!
>AutoPOWER Systems, Fairview, TX (30 mi NE Dallas) Collin County
>QRP-L QRP-ARCI FISTS NORCAL ZOMBIE #522 ARS 10-X 33.2 N 96.6 W EM13RE
>
>Walter Maxwell wrote:
>> >