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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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Old 11-14-2017, 5:11 AM
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Default Do you want to avoid full wave lengths on coax when transmitting?

Will the center conductor capacitively couple to the shielding and then radiate out your feedline? Would you always want the cable to be just off full wave lengths?
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Old 11-14-2017, 6:58 AM
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Use coax that is long enough to reach the load. Using longer only increases the loss and shorter won't transfer much power to the load.
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Old 11-14-2017, 7:41 AM
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Quote:
Originally Posted by beamin View Post
Will the center conductor capacitively couple to the shielding and then radiate out your feedline? Would you always want the cable to be just off full wave lengths?
No and no and no. All of that is old wives tales with zero basis in reality.

The whole point of coax and other feedline is to ensure that RF energy passing through doesn't radiate. The only time that could happen is if you have a bad antenna setup such that it tries to use the feedline as a ground plane. The solution isn NOT to use magic coax lengths, you need to fix your antenna so that it isn't inducing common-mode currents on your feedline. Antenna performance will be crap if you don't, regardless of the coax length.

There is only one rule for coax length: Use the shortest practical length possible to go from radio to antenna. There are some antenna designs that require a common-mode choke to convert from balanced to unbalanced (the j-pole is probably the most common) and that choke can take the form of several wraps of coax at the base of the antenna. But the same principle still applies--use the fewest turns of coax needed to form an effective choke at the lowest frequency you use, to minimize feedline losses.
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Old 11-14-2017, 7:42 AM
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Do you ever see the tower professionals measuring the feeder as they unreel it from the drum? Just saying!
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Old 11-14-2017, 7:52 AM
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The basis for the myth is a quirk in the way SWR meters work. If the coax is an exact odd multiple of 1/4-wavelength electrically (after accounting for the coax propagation velocity factor) the RF reflected back from the antenna will be exactly out of phase with the transmitted signal, and the SWR meter won't "see" it, which causes the meter to give an erroneously low SWR reading.

Some people jumped to the confusion that such a coax length somehow "fixes" the antenna problem, rather than understanding it merely masks it.

If you connect the SWR meter directly to the antenna, you will get a more accurate reading, and see that coax length has no actual effect on antenna efficiency or SWR.
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Old 11-14-2017, 8:50 AM
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What others' commented. And jonwienke went into a detailed post that explains the old wives' tale about coax length.
For accurate readings, place the meter at the antenna.
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Old 11-14-2017, 11:45 AM
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Does a coax cable 'leak?"... all cables will leak something, but for all practical purposes, as far as a ham is concerned, its nothing-
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There are a few little somethings to bear in mind about coax cables, however, especially when it comes to their lengths- such as:
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With line lengths in multiples of 180 degrees ( half wavelegths) the input will be the same as the load- regardless of the impedance of the line- and irregardless of whether that load is resistive, reactive or some of both. In these cases, such lines can be added or removed without effecting any of the operating parameters so long as the line losses themselves are negligible.**
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Rolling out a length of coax is just a normal way of life for anyone dealing in RF. But it is important to keep in mind the above. You can end up tearing out your hair trying to match an antenna if you inadvertently ignore certain coax lengths.
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...............................CF


**The above is of course, calculated based on Velocity Factors.
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Old 11-14-2017, 2:06 PM
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Quote:
Do you ever see the tower professionals measuring the feeder as they unreel it from the drum? Just saying!
Yes and I've done it myself because we wanted the signals that were in phase when they left the transmitter and phasers inside the cabin to be in phase at the antennas at the top of the mast - then you can fit the connector comfortably on the ground instead of hanging in your straps up in the cold breeze.
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Old 11-14-2017, 2:46 PM
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Yes - but with respect, that's a different example from the one being talked about here. Multiple antennas and phasing harnesses or remote phase splitters are a very different usage, where identical feeder length is important for a different reason. A single antenna can be fed with any feeder length. The only time I fitted a horizontally arrayed cardioid antenna package with two feeders, we had a terrible time because while we measured and made them up on the ground, we had different radiuses going around the corners from the pod at the bottom, and by the time we got to the pair of aerials, one feeder had 'grown' making our connections a real pain! I'd forgotten, as had my colleague about double radius curves. we should have started fixing at the top, but we didn't!
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Old 11-14-2017, 10:54 PM
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Paul, I am going to respectfully disagree with you, when you said "A single antenna can be fed with any feeder length (of coax)."
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As I wrote above, 180 degree multiples in a length of coax will definitely effect any attempts to match an antenna to its RF source.
For example; you've an antenna with a input of 3000 ohm. Regardless if the coax cable is 52 ohms, its going to show 3000 ohms at the transmitter. You can adjust/match the antenna to down to 52 ohms, but remember there will be a tight Q at the antenna: Making that 3000 ohm antenna match a 52 ohm line is going to have loading coils etc. sensitive to lots of variables..... its going to be very sharp and very touchy. Anything that changes the antenna's impedance will be seen directly at the transmitter. If there are protective circuits in the transmitter- it will power down-- if not, you will blow your finals.
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Case #2
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Quarter wave (90 degree) sections of coax present a different set of variables
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If the input impedance of a line in odd multiples of 90 degrees is pure resistance, then its output will be pure resistance also. This can be express'd as:
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".............the Characteristic Impedance is equal to the square root of the input impedance times load impedance............"
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This means that if we have two impedances we wish to match, it can be done by connecting them with 90 degree lines, if their characteristic impedance is equal to the square root of their product.
In effect, the 90 degree line becomes a transformer.... and can transform an antenna impedance to a value that will match a given line.
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I won't go into how coax lines lengths can phase/effect multiple antennas.
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________________________________________
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Sorry to take this discussion off onto another plane. What I want to impress, however, is that coax line lengths DO matter.
That being said, be be aware of the above caveats, and avoid some coax lengths.
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......................CF
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Last edited by Coyote-Frostbyte; 11-14-2017 at 11:13 PM.. Reason: .........Wine.............
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Old 11-15-2017, 3:19 AM
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While I understand the physics, the application simply defeats me. if we are talking HF, then the physical accuracy in determining the nodal points is calculable - assuming the precise specs for the cable are known, and the cutting point, as a percentage of the wavelength is not mm critical , but once you move up the spectrum I'm at a loss to understand the cabling process. I have trouble measuring a window to produce results accurate enough for a blind manufacturer to make blinds that fit. The notion of installers unrolling feeder in a field and measuring it, then calculating the length of the connectors once installed on it, and then building the internal jumpers just seems impossible. If you calculate the wavelength you frequently get many decimal points, which at HF are perhaps insignificant, but at UHF?

I'm also thinking that your application has to be different to mine? Are you thinking insulated guyed MF towers? I don't understand where your example impedance of the antenna comes from otherwise? 3000 Ohm makes me think MF and LF, rather than VHF and above, where no antenna is designed with anything other than 50 Ohm terminal impedance, and a close to 1:1 match to the feeder. Have we all thought about totally different applications here? I've no experience whatsoever with broadcast type installations, where the antennas have high impedance, because of their non resonant lengths, I guess.

Is this what you mean or have I got the wrong end of the feeder?

For VHF and above, I can't see a problem with the usual impedance path from antenna to transmitter - have I lost the plot? (Fine if I have)
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Old 11-15-2017, 11:44 AM
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No Paul, I don't think you're missing the point. Its probably me, launching off into the sciences that has muddled the topic.
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For 99% of the time, a random length of coax will work just fine. Heaven knows, I, my guys and gals don't measure off lines when we roll them out- at least we don't in terms of wavelengths. But I am very aware of the physics of those lines (after all, I'm a physicist- I can't help it )
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I know that in the vast vast applications of coax, things like that Half Wavelength senario are rare- but that's science.... they happens. The original questioner asked if he should avoid certain wavelengths- I think the answer is plain.... It's a good idea to keep the physics in mind.
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Will the average ham ever encounter these issues?... Probably not. But if some new antenna is acting quirky, here is something else to pull out of the diagnostic kit.
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Professionally I "shoot' lines all the time- using some really fancy spectral analyz'rs I will run these line at a spectrum of frequencies, looking for those nodes. As a hobbyist ham- are you kidding ?! I just string them out like everyone else.
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My message here is... for those that like analogies--- You can walk thru those dark alley ways 99% of the time 'No Problem.' But be aware that there is a 1% chance of running into the Boogey Men......
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.......................CF
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Old 11-15-2017, 2:39 PM
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The problem wouldn't arise if we didn't use coax cable! Back when transmitters had them tube things and a matching coupler, you had an antenna and open-wire feeder that could have been any random impedance - you tuned the PA stage to the feeder connected right at the back of the transmitter - the feeder became part of the antenna - sometimes a major part. A bonus was you could have a system with an enormous frequency bandwidth - ship antennas covered from below 400kHz to over 22MHz with just a random length of wire and a single wire feed back to the radio room.

Nowadays you have a transmitter with a low impedance output and you have moved the coupler out to the base of the antenna - progress? I'm not too sure.
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Last edited by majoco; 11-15-2017 at 2:43 PM..
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Old 11-15-2017, 11:29 PM
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No Coax cables.. ? !... (lafffing) OMG !... utter chaos!
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Believe me, I am being anything but serious,--- though I really don't want to harken back to the Gold Age of Radio in the 1930's.
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I have been, like forever, a great fan of the random length long wires and an antenna tuner.... I've carried them with me around the world; - and never had a disappointment. I can really appreciate, Martin (Majoco,) what you mean when you laud the old open wire feeds of the maritime services....
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Staying on the topic of feedlines- I have a story about one feedline that I doubt anyone here has used, much less heard about.
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__________________________________________________ ___
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Coyote's Story Time....
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On a certain project in the Far Far North, somewhere along the US/ Canadian border, we had a temporary test camp set at the foot of some incredibly steep little mountains. This was in the winter, and those familiar with the poems of Robert Service, and the lines ".... with the Northern Lights a running wild..." will identify with what the Aurora's were like..... and these Northern Lights were playing havoc with our radio links to the other side of those mountains.
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Some one in our team proposed we try using microwave. It was also proposed we use a passive repeater (a reflector) on the summit of those mountains, (big hills. actually) which tower'd over our camp/base. From up there it was a clear shot, though miles off- to the other end of the circuit.
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Okay.... who'd climb the 'mountain' side in the sub-zero Yukon winter? Fortunately for us, we had some tough characters who step'd forward. But when they learn'd they would have to haul a "bill board' reflector up the 'mountain,' thru ice and snow------- there was, for lack of a better term-- desertion.
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I still thought that a microwave link would be our salvation- and I recalled something I had used years before as a student.... the "G-Line."
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Okay guys, get your minds out of the gutters-- it is anything BUT !......
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A G-line is a feed line also called a Goubau line- It is a form of a microwave waveguide-- and a very simple one-- It uses a single wire and two conical 'launchers' -- The losses at the microwaves are the lowest of anything I have ever seen.... practically Zero.
..... Interested ?... look it up..
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Stringing a wire several hundred feet long up a hill to a corner reflector is a lot easier than lugging a 'bill board' reflector that same distance. Our system used a single run of some # 6 or #10, (I forget) , copper wire, with "launchers" made of sheet aluminium.

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The G-Line, when strung, was attach'd to an ~20 db corner reflector.... and.... Bingo!- communications !
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I don't have any sites to reference this unique feedline system- but I am sure its out there for anyone interested
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This is maybe a deviation from the original topic, but an interesting aside (and for some, possibly an antenna project.)
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..............................CF


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Last edited by Coyote-Frostbyte; 11-15-2017 at 11:42 PM..
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Old 11-15-2017, 11:49 PM
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Ah, yes - surface-wave transmission line. ISTR it was late 60's and we were on 10cm from a Gunn diode transmitter, that's about 3GHz.I can't remember where we played with it now, but it was the length of a short runway, maybe 1300ft. Something to do with measuring the speed of a car along the runway. The hardest part was keeping it off the ground - hard-drawn copper stranded wire is surprising heavy - in the end we supported it on electric fence droppers. I think we couldn't use coax due to the velocity factor. I'll have to dig out some old photos....hmmm.
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Old 11-16-2017, 12:04 AM
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Quote:
Originally Posted by Coyote-Frostbyte View Post
I don't have any sites to reference this unique feedline system- but I am sure its out there for anyone interested
https://en.wikipedia.org/wiki/Goubau_line

I, for one, am a fan of Coyote's Story Time; real world applications in an interesting format.
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Old 11-16-2017, 12:17 PM
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I think I may have some 35mm slides of that G-Line feed- but as I remember them, they were a disappointment... it show'd up no better than a single long wire draping down a bare, snowy cliff face; a little corner reflector antenna far off on the 'summit.'
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We anchored that wire at the top to a massive rock, and used a Jeep at the far end to draw it taut. That procedure is where I wish I had my camera- -------- I think you can guess why....
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The first attempts to draw that wire tight elicit'd all sorts of comments and comparisons about the differences between males and females when it comes to driving 4X4 vehicles.
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"Easy, Lauri..., Eeee Zeeee !!!"....... "Geeeezz IT !!"..... yell'd one of the women engineers (the guys did the 'summit'- the women were handling the 'camp' end of the line.)
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"I said EASY !!"......... *** KA- Pul-linnng !*****................. "OH Sh*T !"
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That wire flew back down the cliff face like a huge spaghetti snake, whipping thru the air and covering the top of the jeep and me in a most unceremoniously, tangled mess.
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The guys all gather'd around me, big grins on their faces, as I extricated myself from the loops of frozen wire.
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""Better let us do that, Boss Lady" they laugh'd.
Now reduced to a mere spectator, the only damage was to my dignity.

______________________________________________
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I thought the wire might need to be support'd, but it apparently didn't. It made a long, graceful loop (once it was *properly* pulled tight... ) down across the rock face. Other than the fact it work'd, I have no idea how well it was preforming... there must be some way to measure a G-Line's stats, but it was too cold to play with beyond that it was working.
A G-Line does tolerates some curves; it acts just like the earth beneath a ground wave signal - (a rough analogy-)--- which directs the wave front over a varying topography.
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Staying on - or trying to- the subject of wave guides, I've some others ... but work is calling me...
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________________________________________
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and A Big Smile Hans !
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.............................CF

Last edited by Coyote-Frostbyte; 11-16-2017 at 12:23 PM..
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Old 11-17-2017, 1:30 PM
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A long wire, a jeep and a bungee cord (or reasonable facsimile thereof)....
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Old 11-17-2017, 1:55 PM
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Smiles:... Yes, we should have known better, and used some sort of a stress reliever. But, like I said, (and not being at all sexist here, oh no- not at all !! ) --the women had charge of the lower end of that wire.... physicists, engineers --it didn't make any difference to the outcome, at the 'camp end' of things...... **
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When the guys took over, they used a 'come-along' hand operated winch to carefully pull it taut.
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Show-off's..........
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.......................CF
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.**we should have been more cerebral... I carry long 2" nylon tow straps in my vehicles at home... why it didn't occur to us to use something with a 'spring' to it then?.... I blame it on the Northern Lights....
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Old 11-17-2017, 2:06 PM
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Quote:
Originally Posted by Hans13 View Post
https://en.wikipedia.org/wiki/Goubau_line

I, for one, am a fan of Coyote's Story Time; real world applications in an interesting format.
Ah, stole my thunder. I found the same article. It is indeed a fascinating subject, and one which will nicely make a unique topic for a January, 2018 club presentation!
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