Open wire coax

[K5UJ]

I think if you have a 1/2 w/l vertical you need the same length radials (that seems logical at least) so I think you got just about the right length Carl.

[W2PFY]

Erry-Tay,

The 4 towers south of Albany are not in a swamp - and three of them are WROW, one (to the north east of WROW ) is the former WQBK 1300 AM. Afaik 1300 has one single tower...

Next week as time permits, I'll try to prove you wrong. If not, I'll eat humble pie, whatever that is

[k4kyv]

I think if you have a 1/2 w/l vertical you need the same length radials (that seems logical at least) so I think you got just about the right length Carl.

I agree.  With the 1/2λ vertical, the maximum displacements currents will appear 1/4λ away from the base, while the feed point at base of  the antenna will be at a voltage maximum and current minimum.  With a 1/4λ vertical, the additional wire needed to lay down 1/2λ radials is not worth the cost, time and effort.

[K5UJ]

So, the currents are at max halfway out (0.25 w) for a half-wave tower.  I always wondered about that and suspected that was the case.   

Next week as time permits, I'll try to prove you wrong. If not, I'll eat humble pie, whatever that is

Terry if you wrong there's nothing wrong with being wrong other than you being wrong     I been wrong more times than I can count.  What's amazing is the know-it-alls out there in ham radio land who just   can't    admit    they    made    a    mistake and hate it when when some dumb little hammy like me calls them out on it.   The bigger the ego the harder the fall.

[W2PFY]

Erry-Tay,

The 4 towers south of Albany are not in a swamp - and three of them are WROW, one (to the north east of WROW ) is the former WQBK 1300 AM. Afaik 1300 has one single tower...

The station listed at 1300 on the AM dial is WGJD direct from the FCC AM QUERY.

The GPS coordinates are  42 35 23.00 N  73 44 37.00 W. Punch that into Google Earth and you will see 6 Towers in a marsh. I have in the past sat right along side of the transmitter and tuned it in. You can hear splatter 30kcs up and down the band indicating that I was at the station.

What is

Afaik

http://www.fcc.gov/fcc-bin/amq?list=0&facid=40768

[Steve - WB3HUZ]

AFAIK - As Far As I Know.

In general, a ground mounted one-half wavelength vertical is less preferred than a one-quarter wavelength one because the far-field pattern is formed over a much larger area. And since all or nearly all of this area is beyond the radials, more loss is incurred than would be with a one-quarter wave vertical. If the ground conductivity is very high (sea water or a salt marsh), a one-half wavelength vertical may be better.

[WBear2GCR]

Yo Terry!

Ha! Ya got me. ROW is between 87 (NYS thruway) and Rt144. I did not know that 1300 (ex-WQBK) had their AM across the river!! The studio and the FM used to be on the single stick just north east of WROW... I assumed they did the AM from the same spot... well swamp or not that spot just doesn't sling much of a signal... better now that it is higher in power... but still Peeeish weak for the most part...

Another case solved.

                    _-_-bear

[Bill, KD0HG]

Here's a pic I took today of one of the "open wire coax" supports at the old KOA Denver tower site. You can see the center insulator for the center conductor and the four ground wire mounts surrounding it. From those pre-Heliax days..Now replaced with buried 4" line. 950 MHz STL antennas on pole in background.

[W2PFY]

It must have been a popular way to get the signal out to the tower back then. This is WGY's with the wires still in place. I'm going to do a road trip in the next few days to see if it's still in use. What could I use to determine if that feed line is active? The other thing to be discovered is how close I can get to the site.

Update: Doing further research it can be seen that the feed-line is still in use. Maybe this is the only one left with this kind of feed-line?

Here is the site where the pictures and the history are provided

http://www.engineeringradio.us/blog/2010/03/history-of-the-wgy-broadcasting-tower/

[k4kyv]

I find this interesting:

GE engineering felt that forging the members of a triangular tower weakens them and was too risky, thus, a square tower was the solution.

They must have made the cross-sectional members out of one solid piece of steel, and feared that bending them a full 120° to form a triangle would weaken them considerably more than bending them only 90° to form a square. Or they might have used angle stock for the vertical members and the same concern would apply to bending the steel to form the angle irons. That could explain why older towers from that era and earlier were nearly always square in shape, when less material would have been needed to construct a triangular shaped tower.

I find that the writer's command of English could use a little improvement; some of what he writes is not perfectly clear in meaning.

I have an early 1950's Gates catalogue.  They list those brackets and insulator assemblies for constructing open wire coax. I remember a station that Timtron worked at up in Maine was still using those when I visited there in the 1970's.  They had installed a cover, using wooden planks, over the entire length of the line to prevent accumulation of snow and ice on the wires. It was painted white, like a wooden fence, and used similar construction.

I have enough wire, and I have entertained the idea of building one of those for my tower, although I am not convinced that the open wire coax configuration works any differently from a regular two-wire open  line. You just have four wires serving as one feeder and one wire serving as the other. If the current in both feeders is not balanced, the line will radiate no matter what geometric configuration is used.

[k4kyv]

Here's a pic I took today of one of the "open wire coax" supports at the old KOA Denver tower site. You can see the center insulator for the center conductor and the four ground wire mounts surrounding it. From those pre-Heliax days..Now replaced with buried 4" line. 950 MHz STL antennas on pole in background.

Bill,

I can't quite make out from the photo what they used for posts for the open wire line.  Are those regular tee-posts used for fencing, the ones that have the bumps along one side for securing the wire?

[Bill, KD0HG]

Here's a pic I took today of one of the "open wire coax" supports at the old KOA Denver tower site. You can see the center insulator for the center conductor and the four ground wire mounts surrounding it. From those pre-Heliax days..Now replaced with buried 4" line. 950 MHz STL antennas on pole in background.

Bill,

I can't quite make out from the photo what they used for posts for the open wire line.  Are those regular tee-posts used for fencing, the ones that have the bumps along one side for securing the wire?

Not fencing T-posts, they're plain old galvanized steel posts, about 10 feet tall x 4", Don.

[k4kyv]

There must be a bunch of those open wire coax bracket/insulator assemblies lying round in a dark closet somewhere, since so many stations used them in the past, but now most have gone to heliax. I'd like to string one up, and compare efficiency with coax and regular two-wire open line.

[W2PFY]

This was taken from the article. What does it mean? I'm not that well versed on some of this stuff.  

The tower was fed with 600 ohm open transmission line, 180 degrees long.

Does this mean that when you use this type of feeder, it would need be a certain length for proper match to the tower? The 180 degrees long is my question.

[K3ZS]

That would be 1/2 wavelength at the operating frequency.     The impedance at the feed point would be the same as at the antenna.

[k4kyv]

You would probably use a matching network between transmitter and  feedline, and maybe another one between feedline and antenna.

If the 4 outer wires are grounded at every pole, as in the example in Bill's photo, the outer wires are being forced to remain at RF ground potential.  Unless each pole has its own little radial system, there would be ground losses there.

If the feedline is run flat, the inner wire would operate without SWR, much as in the case of a single-wire fed Windom.  With a mismatch, there would be standing waves on this wire.

Would just 4 wires surrounding that inner wire be enough to provide the "3rd conductor" with equal and opposite currents to the inner wire, a function served by the inner surface of the braid of regular coax, and effectively shield the inner wire well enough that it would radiate?

[Bill, KD0HG]

Don, I found a book containing an analysis of several types of homebrew unbalanced lines, including the one posted here. It's Radio Antenna Engineering by Laport, dated 1952. Laport was the CE for RCA's International Division. It's an awesome book.

He starts with an analysis of the simplest unbalanced open wire system, which is a single wire connecting a transmitter and antenna. He states: "This design features the utmost structural simplicity, but its use is restricted to short runs and low to medium frequencies or temporary installations. When the line runs over a buried wire ground system, its attenuation may not be objectionable and when not too high above ground, it's radiation is often tolerable."

This evolves to the five-wire configuration we're discussing, of which he states, "Five wire unbalanced feeder with four outer wires grounded"..."This type of feeder is characterized by a high value of k with consequent reduction in earth-return current and overall attenuation. The four grounded wires provide a high degree of shielding for the inner wire." He goes on to describe a sample where the inner wire is .064" diameter, the distance from the center wire to any of the outers is 10", Z= 345 ohms. Formulas for any dimensions are included.

The author states that this sort of setup goes back to RCA in 1938, "..very popular..with wide application in medium frequency broadcasting...where low feedline radiation is essential....and to avoid the expense and complications of concentric line (true coax).

I'll be that RCA made the pictured spreader assemblies.

[W8IXY]

Several years ago (2006)  I was passing by the WGY site and saw several workers up on the tower, so I figured that someone would be on site.  I stopped in and found that I was right.

I asked the engineer on the site about the WGY open wire transmission line and he said that they were using a 3" buried coax on an every day basis, but still maintained the open wire line as a backup.   Now, that was about 4 years ago.

From the time WGAR-AM in Cleveland (50kw into a 5 tower array) went to 50kw in the late 1940's, until the early 70's, they fed an open wire transmission line from the transmitter in the building, out to the phasor at the center tower of their array, and also used open wire line from the phasor to the other 4 towers.  They used the same pattern and power 24/7 so no switching between day and night was needed.  They used a 6 wire "cage" around a two wire feed contained within the "cage".  I think the impedance of that system was 230 ohms.  I can't remember now if the two center feed wires were balanced or fed in parallel.  They used to have occasional transmission line flashovers when a colony of gnats went drifting across it on some warm summer evenings.  Someone with a calculator can figure out what kind of voltage was present with 50kw @ 230 ohms.

Also, WADC/1350, Akron, Ohio, 5kw 24/7 DA-1, (several call sign changes since then), from the 40's probably into the 80's, used a two wire (actually copper tubing) parallel feed system between the transmitter building and the two towers.  There was no cage or shielding....just the parallel run of feedline. 

At the Bethany, Ohio VOA site until they dismantled it a decade or more ago, they also used several unshielded parallel runs of copper tubing between their transmitter building, and out to their huge crosspoint switcher, and from there out to the array of rhombic and Sterba curtain antennas. 

73
Ted W8IXY

[Bill, KD0HG]

Thanks for all the good info, Ted.

That book described that WGAR version. There were two paralleled wires in the center, a few inches apart, with a six wire cage around both. The author analyzed a version made of #14 wire (.064"). Yes, it was in the 200s of ohms and its power handling was astonishing, a couple hundred kilowatts.

I toured Bethany in the early 1970s. I thought that it would be the coolest job in the world to work there. I remember the open wire feeders up on the poles and the knife switch disconnects. I asked about the piles of ceramic and glass shards on the ground, they were the exploded remains of big capacitors that had been up on the poles.

[W2DU]

Yeah Bill, yer right about Laport's book being awesome!! Ed Laport stopped in at the RCA Labs Antenna Lab quite often while I was working there with Jess Epstein (of Brown Lewis & Epstein).

Ed loaned me a copy to let me review it, after which I bought a copy. Forgot to have him autograph it before it was too late before he passed away.

Anyway, I scanned the entire book many years ago, and still have it on my hard drive. So Don, if you'd like a copy of the Laport book I can make you CD copy. In fact, everyone involved with the AM forum should have a copy--it's unique to our mode of operation.

Walt, W2DU

[k4kyv]

I received the CD and wish to publicly thank you, Walt, for your effort.

The book  contains an extensive section on various configurations of open wire line. Laport specifically addresses the topic of this thread, balanced vs unbalanced open wire feeders. He states that an unbalanced open wire line can be two-conductor and even one-conductor (the best example of one-conductor that I can think of is the classic single-wire fed Windom).  Although I have not yet studied the entire text in absolute detail, after one quick read I have inferred that there is always some radiation from an unbalanced line, operating as a flat untuned feeder, but that the overall efficiency is usually satisfactory.  The more wires you add to the zero potential or grounded side, the lower the surge impedance and the better the shielding of the central "hot" conductor, but it never quite equals a true coax line with a solid outer conductor, although it approaches that performance as the number of outer wires is increased.  For extremely long lines, the loss due to radiation would probably be less than the dielectric loss of regular coax. The surge impedance can be lowered further by using two, three or more closely spaced parallel wires as the central or "hot" conductor.

One obvious advantage of this type of feeder is that it can handle a tremendous amount of power at minimal cost, if properly constructed for that purpose.

BTW, for the benefit of anyone else who may be using Walt's CD or a copy of the file, typical of any PDF, the page numbering does not follow exactly the page numbering of the actual book, which can make using the index and table of contents difficult.  The reason the page numbers don't track exactly is that the unnumbered preface pages, cover, etc. count as page numbers in the PDF file.  Also, Walt made a duplicate scan of each page with photos, apparently using black-and-white for the best clarity of the text, and grey-scale for improved resolution of the photos.  This causes the PDF page number to steadily increase beyond the book page number as one progresses from the first page to last.  

I have created a text file on my desktop, to compile a cross-reference between book and PDF page numbers.  As I explore the book, whenever I think of it, I observe the two page numbers and add to the list, and I referenced at the outset the page numbers at the beginning of each chapter as well as the table of contents and index.  Although I don't see a great need to reference every single page, I hope to soon have compiled a complete enough list to make it easy to locate any page listed in the index and table of contents.  When I open the PDF, I also open the text file.  When browsing the book, the page reference is always available with one click at the task-bar.

If anyone else is interested, I can post my list as created so far, and anyone else using the same PDF could add in number references for pages they browse, and by sharing updates to the list, a cross-reference for every page listed in the index and TOC could eventually be created without a tremendous amount of work on anyone's part.

An original paper copy of that book is something to keep a lookout for at hamfests, flea markets and used book stores.

[Bill, KD0HG]

Yes, Don, that *is* one of the best antenna books that I have ever perused. It would be worth reprinting in the, say, ER bookstore. The author is brilliant.

I just found (today) one of those RCA open wire coax assemblies lying on the ground. Missing the big center insulator.

[k4kyv]

I wonder if there would be any copyright or royalties issues for anyone to reprint it and offer it for sale, for profit or not.

I believe there is a clause in the most recent copyright law that material published before a certain date, sometime in the 60s maybe, is considered public domain unless the owners actively took the proper steps to renew the copyright. But for more recent publications, renewal is automatic and copyrights are almost for perpetuity.  Since the original writers and publishers for out-of-print books may be dead or impossible to locate, this is effectively locking a lot of valuable human knowledge away in a vault and depriving the public of the information.

The other thing that is killing a lot of human knowledge is the crappy self-destructive acid paper that it was printed on.  

[K5UJ]

there are 2nd hand copies available from abebooks.com but they are super expensive--$150 to $230.

In another thread, it might have been the one about your L network Don, I posted a link to an online pdf copy of the book.  Someone scanned and digitized it and did a lot of copyright investigating and concluded the book was out of copyright now and in the public domain.

rob

[W2DU]

Rob, that was Dave Platt. IIRC, his call is ae6ao--maybe not quite, but close. He checked with McGraw-Hill, and found out that it is in the public domain. I  copied my copy long before Dave did, but I kept it to myself until I learned from Dave that copying and distributing was ok.

Walt