Open-Wire Feedline Measurements

When investigating 600 ohm ladder line, a few questions come up that I hope some of you can answer. It appears this type of feedline can operate with significant vswr because of its extreme low loss characteristics. On the other hand, impedance matching between the feedline and antenna feed point become important with coax type feedlines. Basically, I am a firm believer of bench-marking feedlines for future reference. Coax is relatively simple to bench mark. Inserting an inline wattmeter at the source of the 50 ohm feedline with the use of a fixed known load at the other end and record the wattage. Remove the wattmeter and place it at the end of the feedline next to the same load and record the wattage again.

When trying to benchmark open-wire feedline for loss, it becomes a little more difficult.

Since most inline wattmeter's are 50 ohms, placing a wattmeter would have to be placed ahead of an impedance matching network matching 50 ohms to 600 ohms and the another network matching 600 ohms back to 50 ohms again? Correct?
Of course the loss in the matching networks would also have to be known.

The other concern is having the ability to easily verify if the feedline currents in an open-wire feedline are 180 degrees out of phase, minimizing any feedline radiation. Thanks for any help here.
 

davenn

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Since most inline wattmeter's are 50 ohms, placing a wattmeter would have to be placed ahead of an impedance matching network matching 50 ohms to 600 ohms and the another network matching 600 ohms back to 50 ohms again? Correct?

Yes, unless you bought/made a wattmeter that had 600 Ohm in and out.

Of course the loss in the matching networks would also have to be known.
yes
 

sophiecentaur

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It is possible to make a directional coupler (one for each direction) with a loose coupling coefficient and use the two output signals to find the reflection coefficient. I have a reference to a successful model, used on a closed wire HF feeder from an HF Broadcast Transmitter. If the feeders are open wire then it is possible to use a trolley, moved along the conductor pair to get a direct VSWR plot. Not something you could put together in five minutes but it would give good answers. These days, you could use an optical link from an isolated probe on the trolley to relay the measurements.
 

jim hardy

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If the feeders are open wire then it is possible to use a trolley, moved along the conductor pair to get a direct VSWR plot.
I second sophie's motion.
We did just that in my high school electronics class ca 1963, under tutelage of a natural born teacher.
It's a great exercise for learning . Memories of it are what got me through my undergrad antennas course because i could relate each term in that fearsome equation to something i'd observed and quantified years earlier before knowing the relevant math.

I recommend it to any student of the craft.

old jim
 

sophiecentaur

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I second sophie's motion.
That's because we both go back about the same distance in time. That sort of Heath Robinson experiment would be too expensive and time consuming for a modern School exercise, I suspect. The same goes for the Slotted Line work which is also labour intensive. I ask myself how a posh Network Analyser is enough to give any beginner the right feel for transmission line behaviour. And then there's the dreaded Simulation Software which is indistinguishable from a computer game display unless it's treated with both respect and skepticism.
PS @jim hardy take a glance through the link I posted about measuring two wire closed HF feeder. Bob was a brilliant colleague of mine and his name still ranks high in BBC Engineering circles. (RIP)
 

tech99

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When investigating 600 ohm ladder line, a few questions come up that I hope some of you can answer. It appears this type of feedline can operate with significant vswr because of its extreme low loss characteristics. On the other hand, impedance matching between the feedline and antenna feed point become important with coax type feedlines. Basically, I am a firm believer of bench-marking feedlines for future reference. Coax is relatively simple to bench mark. Inserting an inline wattmeter at the source of the 50 ohm feedline with the use of a fixed known load at the other end and record the wattage. Remove the wattmeter and place it at the end of the feedline next to the same load and record the wattage again.

When trying to benchmark open-wire feedline for loss, it becomes a little more difficult.

Since most inline wattmeter's are 50 ohms, placing a wattmeter would have to be placed ahead of an impedance matching network matching 50 ohms to 600 ohms and the another network matching 600 ohms back to 50 ohms again? Correct?
Of course the loss in the matching networks would also have to be known.

The other concern is having the ability to easily verify if the feedline currents in an open-wire feedline are 180 degrees out of phase, minimizing any feedline radiation. Thanks for any help here.
The losses from 600 Ohm line are extremely small unless you have a very long length, so measurement technique will need to be good. Does it have to be 600 Ohms? I think you could obtain a pair of 50 to 450 Ohm toroidal baluns with good performance, which you can test back-to-back.
 

jim hardy

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PS @jim hardy take a glance through the link I posted about measuring two wire closed HF feeder. Bob was a brilliant colleague of mine and his name still ranks high in BBC Engineering circles. (RIP)
... i dont know quite where to find it - did i miss it in this thread?
 

dlgoff

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@jim hardy and @Baluncore A quick slightly off topic question.
I putting up a 40 meter dipole antenna using a ladder wire feed. What are the best ways to get the ladder wire physically into the home?

Thanks
 

tech99

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@jim hardy and @Baluncore A quick slightly off topic question.
I putting up a 40 meter dipole antenna using a ladder wire feed. What are the best ways to get the ladder wire physically into the home?

Thanks
One suggestion is to use two coaxial cables side by side for a short distance.
 

dlgoff

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One suggestion is to use two coaxial cables side by side for a short distance.
Thanks for the suggestion. What do you do with the shields? Attached to the chassis of the radio?

edit: Or maybe leave unconnected?
 

tech99

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Thanks for the suggestion. What do you do with the shields? Attached to the chassis of the radio?

edit: Or maybe leave unconnected?
I suggest connect the shields together and leave floating. But this question will cause hours of argument.
 

sophiecentaur

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@jim hardy and @Baluncore A quick slightly off topic question.
I putting up a 40 meter dipole antenna using a ladder wire feed. What are the best ways to get the ladder wire physically into the home?

Thanks
It's an interesting problem.
How important is the comparatively low loss of the ladder feed? Is it really preferable to using coax with a balun at the antenna feed? Coax is extremely convenient (and well behaved) and the loss at HF is not bad for mid priced cable at legal powers.
But enough of the typical PF 'do it another way' response.
This is not a 'minor construction' so I assume you could manage to feed through a handy window with a hole drilled through the glass to isolate the feeder from the room. With a relatively high impedance balanced line the route should be taken so that the feeder is held away from other conductors and that the plane of the feeder (locally) is parallel to nearby surfaces (to maintain balance). Your TX / RX should be connected as near to the way-in as possible. The TX connection needs to be pretty symmetrical if you want to avoid RFI due to imbalance.
 

dlgoff

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I suggest connect the shields together and leave floating. But this question will cause hours of argument.
Okay. Floating it is. Thank You. [/end offtopic]
 

dlgoff

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It's an interesting problem.
How important is the comparatively low loss of the ladder feed? Is it really preferable to using coax with a balun at the antenna feed?
This is an old antenna and feed wire I had as a youngster. Thought I'd use it to feed an old ventage receiver (no TX).
With a relatively high impedance balanced line the route should be taken so that the feeder is held away from other conductors and that the plane of the feeder (locally) is parallel to nearby surfaces (to maintain balance).
Since I live in a metal box (moble home :oldcry:), I think the thru-wall coax as @tech99 suggested will do. Thanks for this useful info.
 

Baluncore

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What are the best ways to get the ladder wire physically into the home?
One way to get an open-wire twin-feed into the shack is through two small holes in a window. You can drill the glass with a diamond drill or replace a small pane of glass with a sheet of polycarbonate and drill that. Use threaded rod with rubber washers to connect the wire lines through the holes, and to seal against the weather.
 

sophiecentaur

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You can drill the glass with a diamond drill or replace a small pane of glass with a sheet of polycarbonate and drill that.
That was true in the old days. Modern double / triple glazed units are a harder problem - although i'd bet that a hole could be made through the plastic glazing bar that's well away from the edge of the window.
 

jim hardy

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I was going to suggest a variation on that theme
short length of PVC pipe through a wall,
large enough to encircle the two conductors,
pipe cap on each end drilled at same spacing.
Seal outside with foam and put a rain deflector made of aluminum flashing over it, secure with sheet metal screws.

Won't look too bad.

old jim
 

sophiecentaur

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I was going to suggest a variation on that theme
short length of PVC pipe through a wall,
large enough to encircle the two conductors,
pipe cap on each end drilled at same spacing.
Seal outside with foam and put a rain deflector made of aluminum flashing over it, secure with sheet metal screws.

Won't look too bad.

old jim
I was concerned that this solution may introduce too much parasitic capacitance - which is why going through a window, well away from walls / lintels / mains wiring would be preferable. But we should be specifying the tolerable level of parasitics before choosing a solution.
 

jim hardy

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I was concerned that this solution may introduce too much parasitic capacitance - which is why going through a window, well away from walls / lintels / mains wiring would be preferable.
I too worry about the open line. Were it 300 ohm TV Twin Lead i'd use a thee inch PVC conduit figuring 10X distance between conductors should be adequate to minimize the impedance discontinuity.

Don's house (been there) has aluminum siding .

When tinkering with my TV antenna i put PVC conduit in the microwave oven so see if it reacts with RF energy. It neither got hot nor made sparks.
So i figured it might make a safe "tunnel" for that EM field between the conductors of his ladder wire line.

But who knows - he may need 6" diameter !

Our electricl penetrations in the power plant were similar concept - stainless steel pipes through the containment wall with glass to copper seals around each conductor both inside and outside ends. You could see them clearly with a TDR machine , even on the coaxial cables..

old jim
 

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