On Sun, 15 Jun 2014 14:17:00 -0500, "Tim Williams"
<tmoranwms@charter.net> wrote:
>"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
>news:l90qp9t860iq49skck0jmftqoris0jet7g@4ax.com...
>> Only works at low frequencies. A large diameter open wire "ladder"
>> transmission line would probably work equally well, but would require
>> very big baluns at both ends.
>Well, it would presumably work up to the cutoff frequency, limited by
>competing coaxial waveguide modes. And possibly degenerate modes involving
>wire-to-wire coupling, which shouldn't couple strongly to differential mode
>energy, but are nevertheless present. Guessing from the picture, it might
>be good up to... 10 to 100MHz?
I like order of magnitude guesses. I don't think I can do better.
Let's see if I can still function after eating my own cooking.
Looking at the photo at:
<http://upload.wikimedia.org/wikipedia/commons/6/61/Solec_Kujawski_longwave_antenna_feeder.jpg>
there's not much in the way of known references from which to extract
dimensions. The best I can do is the roadway, which I would guess(tm)
is about 2.5 meters wide. Translating the track width to the square
metal frame and adjust for parallax, perspective, and indigestion, I
get 2.0 meters for the shield diameter and 0.4 meters for the center
conductor. (5:1 diameter ratio extracted from my monitor with a
ruler, which I think is fairly accurate).
For air dielectric:
Z = 138*log(D/d) = 138*log(5) = 96 ohms
I'll call it 100 ohms.
The original photo at:
<http://en.wikipedia.org/wiki/Transmission_line#Coaxial_cable>
indicates that it's 225 KHz at 1200 kW.
Power = E^2 /R
E = sqrt(Power*R) = sqrt(1.2*10^6 * 100) = 11 kV volts RMS. Peak
voltage (which is what makes the arc) will be 1.4*11kV = 15kV.
At 3x10^6 V/meter, a 0.8 meter gap will flash over at 2.4 kV.
So either my guesswork and math suck (a likely possibility), or this
thing isn't going to work at rated power. (I really hate it when
reality doesn't agree with my reverse engineering).
Here's the station and tower:
<http://en.wikipedia.org/wiki/Longwave-transmitter_Solec_Kujawski>
More photos including better pictures of the open wire coax
arrangement:
<http://radiopolska.pl/wykaz/pokaz_lokalizacja.php?pid=610>
Google street view works, but doesn't get close enough to the towers
to show details:
<http://maps.google.com/?t=h&om=0&ll=53.022778,18.261111&spn=0.023387,0.058537>
>I think the downside of ladder line would be the prohibitive wire diameter,
>even for a high impedance (widely spaced wires) line. And too wide a
>spacing and you start having to worry about unwanted radiation.
Agreed. If we accept my 100 ohm impedance guess(tm), the current in
the coax would be:
P = I^2 * R
I = sqrt(P/R) = sqrt(1.2*10^6 / 100) = 110 amps
However, that's distributed equally among the individual wires. I
count 12 wires each in the shield and center conductor or about 9 amps
per wire. No problem there.
>(They could
>go one further and twist it every half wave, so the far field is nulled.)
That's what the old telegraph wires did to minimize crosstalk.
>But yeah, the less balun you need at those power levels, the better.
Yep, that's what's nice about using coax cable. If the antenna and
transmitter impedances were fairly close to my speculated 100 ohms,
the baluns are not needed.
>Interesting to note, the voltage on the outer cage (with respect to ground)
>is probably nonzero. Or if it is, it's because they did indeed add a common
>mode choke, or something along those lines. Thought being: the ground
>impedance is nonzero, so the "ground" at the antenna feedpoint will have
>nonzero voltage.
I beg to differ. If you look at the photo:
<http://upload.wikimedia.org/wikipedia/commons/6/61/Solec_Kujawski_longwave_antenna_feeder.jpg>
the coax shield wires appear to be directly connected to the metal
frame. If they were isolated, insulators would have been needed.
Methinks the outer cage is at ground potential.
>> I'm also wondering about the losses. At low RF frequencies, skin
>> depth and resistive losses predominate. I don't think there's enough
>> copper in the wires to be really low loss. I've built broadband wire
>> cage dipoles where such losses almost became a problem. At megawatt
>> power levels, any transmission line losses can be a serious problem.
>Hmm, not obvious if they used litz -- suppose that's a possibility.
>Probably not worth it though.
Litz wire would certainly have helped a little, but I suspect the cost
and maintenance problems might have been prohibitive. Besides, if
you've got 1.2 Megawatts to play with, what's a few extra watts of
loss?
>I would guess the impedance is high enough not to be a big problem
>(obviously, they would've thought of that; question is, we want to know
>why). Seems like it would be around 50 ohms if it were solid coax, but
>since it's not solid, it's probably higher. But by how much? 100 ohms?
>300 ohms?
See my previous guesswork. I would say 100 ohms.
>Still quite a few amperes at that power level, either way.
110 A RMS total or 9 A per wire.
VOA made use of open wire transmission lines at high power levels:
<http://cryptome.org/eyeball/voa/voa-antennas.htm>
<http://www.lbagroup.com/blog/wp-content/uploads/2010/08/open-wire-antenna-transmission-lines.jpg>
>If the load is a standard 1/4 wave tower, it would match near 50 ohms, which
>would require a matching network or transformer, which is again undesirable.
1/4 wave at 225 KHz is 333 meters (1100 ft). If the tower had a
capacitive hat, it could be made considerably shorter. The web site
says 289 and 330 meters.
<http://en.wikipedia.org/wiki/Longwave-transmitter_Solec_Kujawski>
A 1/4 wave monopole over a decent ground works out about 35 ohms
impedance. That's not going to match very well to a 100 ohm feedline.
I couldn't find much techy detail on the station, so I have no clue
how their doing the matching. A clue is that there are small brick
building at the base of both towers, which suggest a matching network.
>Given the potential for ground currents, they may well have a transformer
>out there anyway, in which case it doesn't matter much what the line or load
>are; they just tap them off as needed.
There are going to be ground currents anyway. A monopole requires a
buried counterpoise in the form of ground radials in order to form the
other half of the dipole.
>And lightning isolation, might be a plus.
The antenna is in Poland (and my family is from Poland). I suspect
that using cheap electrical wire and just replacing the wire when it
gets hit by lightning, might be all that's required.
>> I wouldn't worry (much) about the wind. The wires should all swing in
>> unison. For wide gaps, spreaders can be added. However, I suspect
>> the real danger is some bird landing inside the cage, resulting in an
>> RF arc barbequed bird.
>
>>:-D
I once designed an antenna that included weight loading measured in
standard bird loads, which I vaguely recall was based on the number of
Gooney Birds (albatross) that could be lined up on the antenna radials
times 10Kg per bird. I took exception to the spec claiming that it
was an unlikely possibility, and was presented with a photo showing a
rather large number of birds neatly lined up on a seriously sagging
antenna element. I didn't protest any further.
>Tim
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558