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Author Topic: resonant antennas and open wire line  (Read 46735 times)
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ab3al
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« on: December 24, 2009, 07:44:18 PM »

Here is one for the x perts here

The idea is to construct a resonant diaperpole for each band 80 40 20 possibly 160 and feed it with either 450 window line or home made 600 open wire.
I will bring the lines to the shack and switch it there.

Being the lazy sort of gen xer that i am I would like to match this in a way that i will take advantage of the fact that the antenna is resonant and not have to go through a tuner.  Is there a way to match the low imp diaperpole to the ladder line and then the ladder line in the shack to the rig with no moving parts.
Basically use it as if the antenna was a co ass fed resonant.

things i cant rap my head around.

1) what load is seen at the end of say 100 ft of 450 line with a 80m resonant dipole at the shack end
2) could a single broadband network be used in the shack for the same antennas on 20 40 80 160
3) would the losses in such a network defeat the purpose
4) would it be so much work design and planning that its not worth it.

the reason for this is the g5rv although it works great  the coax losses (100 ft) are steep and on some bands the feedline radiation (coax) is so high that the tvi is scarry
I know I know 130 ft flattop balance line and a link coupled tuner... but thats the easy way out.. I wanna try something new.
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ab3al
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« Reply #1 on: December 24, 2009, 08:09:51 PM »

nevermind.. just answered it myself 

would need either a delta or gamma match at the antenna and then a 9:1 transfromer and a 1:1 balun  at the feed.  a gamma match would not be practical on a wire and the delta would skew the radiation patter quite a bit... Link coupled tuner seems to be the best solutuion.

anyway Ive gotta run been working on a new wheel thats gonna be better than the old one
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WA1GFZ
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« Reply #2 on: December 24, 2009, 08:12:54 PM »

That would be a job for Santa
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KF1Z
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Are FETs supposed to glow like that?


« Reply #3 on: December 24, 2009, 09:05:32 PM »

Are you talking about 3  seperate dipoles, with 3 independant feeds?

If so, can't you just make each feedline an even multiple of a 1/2 wave, and trim.. ?

The xmitter end of the feedline would just "mirror" the impedance of the ant... right?


OR take the easy way out, and use a JJ supa tuna   Grin
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N2DTS
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« Reply #4 on: December 24, 2009, 09:29:03 PM »

There is NO reason to use open wire line on an antenna with little or no swr.

Its only when there is a lot of swr on the feedline that open wire line pays off.

Brett
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Ed/KB1HYS
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« Reply #5 on: December 24, 2009, 11:52:53 PM »

Regardless of type of feed line, go with the electrical half wavelenght of the resonant frequency the dipole is cut for. The impedance at the transmitter end will be around 72 ohms same as the dipole feed. 

Make sure you take the velocity factor of the line into consideration. OW is fairly high, ~0.85 or so depending.   

There will still be a mismatch at the antenna/feed line junction, unless you are using 75 ohm line that naturally matches a dipole (well it'll be close enough).   

Normally there's no need to worry about the mismatch at the ant/feedline junction. But if you want you can use any type of well designed transformer for that purpose. (for single band antennas aircore balun types work well and tend not to saturate like ferrite based designs. Also not as heavy.)
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73 de Ed/KB1HYS
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ab3al
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« Reply #6 on: December 25, 2009, 10:13:10 AM »

Are you talking about 3  seperate dipoles, with 3 independant feeds?

If so, can't you just make each feedline an even multiple of a 1/2 wave, and trim.. ?

The xmitter end of the feedline would just "mirror" the impedance of the ant... right?


OR take the easy way out, and use a JJ supa tuna   Grin

yes three different ants.

are you saying trim the feedline for a match or the ant
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ab3al
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« Reply #7 on: December 25, 2009, 10:23:29 AM »

There is NO reason to use open wire line on an antenna with little or no swr.

Its only when there is a lot of swr on the feedline that open wire line pays off.

Brett


cost Weight and power

the ant supports here arenet very strong.. I live in a row home with a 20x90 back yard.  + 10 ft to the peak of the roof.

the roof end is a par of crap shack tripods and 12 ft masts 18 ft apart.. total height 45 ft
the far end are 2 50 ft telescoping masts and a 45 ft a little closer in.

I was a little concerned with running qro am through rg8x mini.. flex radio and several henrys 2kx 3k 5k
ther is no center support for the dipole as i have to keep things as un obvious as possible
I recently bough out my competition and ended up with about 6 miles of strand and solid electrical wire to make ants and feed with

anyone know how much it would take to fry rg8x feeding a resonant ant 1.5 to 1 or less

the rg8xmini  would only be about 60 ft long to a central point  ( the shed ) where i have 3 runs of 1 inch hardline (friends in the cellphone industry gave me several 120-150 foot leftovers)
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k4kyv
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Don
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« Reply #8 on: December 25, 2009, 10:57:00 AM »

... the g5rv although it works great  the coax losses (100 ft) are steep and on some bands the feedline radiation (coax) is so high that the tvi is scarry

Then something must be wrong with that G5RV. There is no reason for feedline radiation, either the coax part or open wire line part.  Feedline radiation means excessive common mode currents on the feedline.  If the antenna is balanced as it should be, the feedline radiation should be negligible.  Tvi is most likely a result of poor harmonic suppression at  the transmitter.

Open wire line may be  run as a  flat, untuned line or as a resonant feedline with SWR. One reason to  run open wire at low SWR would be if you wanted  to maintain balanced feeders. Even at 1:1 SWR, the losses in open wire line are much less than those of coax.  Flat open wire feeders would be useful, for example, for a very long feedline.  I measured 8% power loss in a 140 ft.  run of RG-214 "low loss" coax on 160m, at near 1:1 SWR. I'm sure a flat section of open wire line  running at 1:1 SWR would show less loss than that.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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« Reply #9 on: December 25, 2009, 11:40:22 AM »

Strange.  My G5RV put a little rf into the shack but I added a W2DU choke balun where the coax meets the ladderline and that was history.  Also, it now easily loads up on all bands 160 - 6 meter.   Of course I know it's silly to try anything phone on 160 due to it not being very efficient, it does load flat with the tuner.  30 meters also loads now where it wouldn't before the balun was installed.  Before the balun, different bands and frequencies would kick my usb keyboard and mouse right offline, but this doesn't happen anymore either. 

Nevertheless, I'm planning to build a k1jj style link coupled super tuner and play with more copper than just 102'.

Merry Christmas everyone!
KX5JT
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ab3al
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« Reply #10 on: December 25, 2009, 11:55:49 AM »

errr um sorry for the miss speak.. actually what is happening is on 40 meters anywhere on the band with anything over 100w i flicker every cfl within 800 feet of me.. one night for fun i ran a long cord for my keyer to the front porch.. the wife and i sat outside and watched the lights up and down the street indoor and out door blink in time with my cq call.. funny as hell.

don the line loss was not actually measured by me but taken from cebik who did the work at the end of a piece of 100ft 213  was about 10 db feedline loss on 80.

just weighing the options right now to make everything as efficient as possible.

spectrum analizer shows everything is good and clean at the output of the amp
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KF1Z
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Are FETs supposed to glow like that?


« Reply #11 on: December 25, 2009, 12:13:41 PM »

Are you talking about 3  seperate dipoles, with 3 independant feeds?

If so, can't you just make each feedline an even multiple of a 1/2 wave, and trim.. ?

The xmitter end of the feedline would just "mirror" the impedance of the ant... right?


OR take the easy way out, and use a JJ supa tuna   Grin

yes three different ants.

are you saying trim the feedline for a match or the ant


Trim the feedline, same as you need to for a G5RV.
(of course this sort of implies to calculate the electrical 1/2 wavelengths in even multiples, and cut it a little long, so you can trim it down)

This is where a MFJ 259 or Autek RF-1, or VA-1 come in super handy..


You can't really trim an antenna to 'match it'.
Once the ant is cut for the freq you want it, leave it alone, if you trim it, it is no longer resonant for the freq you want..


But, I'm not sure if having 3 or 4 seperate mono-band dipoles is going to gain you much over a single doublet, fed with open wire line, and a good link-coupled tuner to cover all the bands.

One good reason for seperate dipoles is band vs. direction.
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N2DTS
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« Reply #12 on: December 26, 2009, 12:09:00 AM »

I have run 700 watts carrier, about 3kw pep into the stuff for years without any problems...

Brett


[/quote]

cost Weight and power

the ant supports here arenet very strong.. I live in a row home with a 20x90 back yard.  + 10 ft to the peak of the roof.

the roof end is a par of crap shack tripods and 12 ft masts 18 ft apart.. total height 45 ft
the far end are 2 50 ft telescoping masts and a 45 ft a little closer in.

I was a little concerned with running qro am through rg8x mini.. flex radio and several henrys 2kx 3k 5k
ther is no center support for the dipole as i have to keep things as un obvious as possible
I recently bough out my competition and ended up with about 6 miles of strand and solid electrical wire to make ants and feed with

anyone know how much it would take to fry rg8x feeding a resonant ant 1.5 to 1 or less

the rg8xmini  would only be about 60 ft long to a central point  ( the shed ) where i have 3 runs of 1 inch hardline (friends in the cellphone industry gave me several 120-150 foot leftovers)
[/quote]
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W1VD
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« Reply #13 on: December 26, 2009, 07:22:55 AM »

If you go with 'small' coaxial cable consider mil RG-142...double silver shield / teflon insulation / solid silver inner. Will take serious power and is about the same size as RG-58.
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k4kyv
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Don
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« Reply #14 on: December 26, 2009, 08:26:06 AM »

There should not be any feedline radiation on the coax only on the window line part and it will be very high at that point, but it must have a good heavy balun where the coax connects to keep it on the window line and off of the coax.
 

Have I missed something re the G5RV?  As far as I can tell it is basically a slightly shortened dipole fed at the midpoint with open wire tuned feeders, which are then fed directly by a piece of coax line at a specific point where the transmitter end of the OWL presents a reasonably good match directly to the coax without a tuning network. Therefore, there should be no unbalance nor any radiation from the open wire line.  If the coax is looking at a mostly resistive load somewhere near its surge impedance, there should be low SWR on the coax, and little or no common mode current. This is not unlike the common practice of using coax to feed a balanced half-wave dipole directly at its midpoint without a balun, which often results in a good SWR and minimal common mode current on the outer surface of the braid.

Some people have even told me that part of the feedline of a G5RV is supposed to radiate, much like a vertical antenna, to give low angle radiation good for DX.  I always took that to be nothing more than some kind of Hammy Hambone urban legend.

So, could someone explain what causes the open wire part of the feedline to radiate?
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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« Reply #15 on: December 26, 2009, 10:27:10 AM »

Quote
I measured 8% power loss in a 140 ft.  run of RG-214 "low loss" coax on 160m, at near 1:1 SWR. I'm sure a flat section of open wire line  running at 1:1 SWR would show less loss than that.

That way makes it sound worse than it really is since the actual loss is only .25dB/100' at 2 mHz. Or put into a visual mode it takes 5-6dB per S unit.

With RG-8 foam it is .2dB and with a good RG-11 CATV foam cable it is only .1dB. If those sort of losses bother you then you have too much time on your hands Grin

Since I only use the RG-11 CATV cable and 75 Ohm hardline I dont have to continually lose sleep over losses, even with 400' to the 160/80M inverted vees.

For a dipole, use quad shielded and flooded RG-11 foam. Its light weight and with a balun there will be no feedline radiation even with a fairly high VSWR. It will handle all the power you can muster up with those Henry's.


Quote

don the line loss was not actually measured by me but taken from cebik who did the work at the end of a piece of 100ft 213  was about 10 db


That was your first mistake Roll Eyes

Carl
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« Reply #16 on: December 26, 2009, 11:30:07 AM »

I've seen a station make a 15 minute AM  OB test xmission on 75M using matched RG8X using xx KW pep... Grin   (into a dummy load of course)  The coax didn't even get warm. Never wud have believed it.   Coax can take a lot of power at the low freqs if reasonably matched.

Over here I have several 600' runs of 75 ohm  aluminum hardline to the tower/ antennas. I don't worry about losses that add up to less than a db.

T
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« Reply #17 on: December 26, 2009, 11:40:26 AM »

Derb, looking at that aerial arrangement. If you want to play on 160M then I would suggest something along the lines of a folded unipole. Phil K2PG has been running one for years and he does quite well in the postage stamp lot he has in NJ.
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« Reply #18 on: December 26, 2009, 08:52:43 PM »

I witnessed 2500W of A.M. carried by RG8X and it got warm. This was a 50 ohm antenna system tuned for 3.880 at a station in the N.E.
Fred
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Steve - WB3HUZ
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« Reply #19 on: December 26, 2009, 09:56:38 PM »

You have it right Don. Anyone who claims the feeders are supposed to radiate doesn't know what they are talking about.


There should not be any feedline radiation on the coax only on the window line part and it will be very high at that point, but it must have a good heavy balun where the coax connects to keep it on the window line and off of the coax.
 

Have I missed something re the G5RV?  As far as I can tell it is basically a slightly shortened dipole fed at the midpoint with open wire tuned feeders, which are then fed directly by a piece of coax line at a specific point where the transmitter end of the OWL presents a reasonably good match directly to the coax without a tuning network. Therefore, there should be no unbalance nor any radiation from the open wire line.  If the coax is looking at a mostly resistive load somewhere near its surge impedance, there should be low SWR on the coax, and little or no common mode current. This is not unlike the common practice of using coax to feed a balanced half-wave dipole directly at its midpoint without a balun, which often results in a good SWR and minimal common mode current on the outer surface of the braid.

Some people have even told me that part of the feedline of a G5RV is supposed to radiate, much like a vertical antenna, to give low angle radiation good for DX.  I always took that to be nothing more than some kind of Hammy Hambone urban legend.

So, could someone explain what causes the open wire part of the feedline to radiate?
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« Reply #20 on: December 27, 2009, 11:56:28 AM »

You have it right Don. Anyone who claims the feeders are supposed to radiate doesn't know what they are talking about.

 


Be careful about pointing out the fact that those who don't know what they are talking about don't know what they are talking about.

It upsets touchy feely types.

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« Reply #21 on: December 27, 2009, 12:51:10 PM »

Since certain touchy feely types like to base their claims on hearsay or antenna computer simulators check out the G5RV simulator screenshot attached below.

Notice the matching section's vertical polarization on both 40 and 80 meters?

 Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin


Er uh. Sorry I don't do any modeling myself just lots of real antennas. When I want something modeled I ask da boys to do it for me.

Am I reading your screenshot correctly or are you stating that -73 dBi represents vertical polarization?
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k4kyv
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« Reply #22 on: December 27, 2009, 03:26:27 PM »


Just go to some of the British websites. There's a billion websites on the G5RV and each one says something different. That screenshot was just one page from a complete G5RV evaluation from the radio organization over in England.

I read another article yesterday that said the matching section must be exact and its bottom must be at least 1m above ground for the lowest frequency of operation otherwise the antenna will radiate vertically on those lowest two bands.

With all of the ready-made G5RV’s available today there is no telling if they are all made correctly or not. Did they use 450-ohm window line or 390-ohm line? Were the right calculations made, etc., etc., etc. If you built it yourself, what calculations did you use? It’s actually kind of scary.

It's beginning to sound a  lot like audiophoolery.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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« Reply #23 on: December 27, 2009, 03:49:13 PM »

Er uh. Sorry I don't do any modeling myself just lots of real antennas. When I want something modeled I ask da boys to do it for me.

Just go to some of the British websites. There's a billion websites on the G5RV and each one says something different. That screenshot was just one page from a complete G5RV evaluation from the radio organization over in England.

I read another article yesterday that said the matching section must be exact and its bottom must be at least 1m above ground for the lowest frequency of operation otherwise the antenna will radiate vertically on those lowest two bands.

With all of the ready-made G5RV’s available today there is no telling if they are all made correctly or not. Did they use 450-ohm window line or 390-ohm line? Were the right calculations made, etc., etc., etc. If you built it yourself, what calculations did you use? It’s actually kind of scary.

A properly designed G5RV installed at the right height above REAL ground will probably work correctly. That probably rules out a few of the ready-made store-bought kind.

So would you care to answer my question?

I don't need to do calculations to look for feedline radiation which only exists in your mind.
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« Reply #24 on: December 27, 2009, 10:42:22 PM »

I used to run a home made g5rv, compaired to a resonant dipole fed with coax, it was a poor and noisy performer on 80.
If the open wire line was not the right length, the tuner had a hard time tuning it at a high voltage point on 80 meters.
My coax run was short, using RG214, about 25 feet of it.

Brett
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