Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Some words on the Ladderline vs. Coax issue...




 
The AM Forum
October 19, 2019, 01:23:18 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: 1 ... 3 [4] 5   Go Down
  Print  
Author Topic: Some words on the Ladderline vs. Coax issue...  (Read 55488 times)
0 Members and 1 Guest are viewing this topic.
K3ZS
Member

Offline Offline

Posts: 1036



« Reply #75 on: November 17, 2009, 11:49:15 AM »

When I put up my 390 ohm a brown crap window line a put a twist every few feet.   Most pictures I see using real ladder line have long sections running without a twist.    I wonder if that could have some effect with W5HRO's noise pickup.   I think having a twist would reduce pickup and feedline radiation.
Logged
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #76 on: November 17, 2009, 12:37:00 PM »

Shane, Vertical radiation is not possible from my Open wire line. Thats been pointed out many times.  I think walt explained the gain on 40 meters.

C
Logged
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #77 on: November 17, 2009, 01:08:41 PM »

I dont have to do that. The line and tuner is balanced. 

C
Logged
KM1H
Contributing
Member
*
Offline Offline

Posts: 3519



« Reply #78 on: November 17, 2009, 01:47:00 PM »


To the locals on 40 meters I gained 20!!! DB.  These guys use Back yard verticals.  On other bands I cant honestly tell the difference.  Why the huge gain on 40? 


This is a VERY simple question to answer, Clark.

Cross polarization = approximately 20 dB.

Polarization would make a big difference for local ground-wave coverage, but ionospheric propagation is so turbulent that the polarisation of the signal becomes random at the receiving end.

As for the balanced line coming off the antenna at 90°, I say if everything is truly balanced, it doesn't make any  difference whether the feeders run horizontally, vertically or slanted as long as they remain perpendicular to the antenna. If the antenna and feed line are indeed configured symmetrically, there would be negligible coupling between the radiating antenna and feeders.  Nearby metallic objects such as plumbing, overhead electrical wiring, house wiring, smokestacks, etc may upset the balance, but if there is reasonable separation between antenna system and stray objects, those unbalances would be too negligible to worry about.


If we stick to Clarks question and dont go drifting off the subject, a vertical component is a plausible explanation that nobody has presented an acceptable refute.

Its not so much as a polarization issue as it is where the azimuth lobe is situated. With the horizontal at that height there is minimal radiation at the low angles so the local signal is attenuated. With the different feedline there is sufficient unbalance to permit some radiation at a lower angle. Im certain it can be computed with NEC analysis which would show a substantial current (power) node on the feedline. It is the at current node where the maximum radiation takes place. That antenna system has multiple current nodes.



Walt, with all respect, I suggest that you include a series of NEC-4 plots along with that G5RV article using the design exactly as described by Varney.  I too have read of the disagreements and supposed analysis using software. However when working with OW the analysis accuracy of many commercial or free products are not up to the task.  You are old enough  Grin  to remember the current/voltage distribution curves of the antennas and feedlines in the various handbooks as well as engineering books which I believe is still accurate today Cool.  Almost all admonished about feedline radiation as well as the necessity of having the feedline drop 90* vertically to minimize radiation. Why the revisionists today want to rewrite these standards is beyond me.

Although I rarely view QST as a bastion of engineering excellence a comment in the December issue caught my eye. "According to the scientific method, a theory must be tested to confirm its validity". Some of what I read on here and elsewhere on the subject neglects those basics.  And dont get me going on Cebik!

Carl
KM1H

Logged
KM1H
Contributing
Member
*
Offline Offline

Posts: 3519



« Reply #79 on: November 17, 2009, 01:49:34 PM »

I dont have to do that. The line and tuner is balanced. 

C


And the absolute proof is??

Or maybe its just the operator that is unbalanced Roll Eyes  Couldnt resist that one Grin

Carl
KM1H
Logged
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #80 on: November 17, 2009, 02:12:14 PM »

LOL....    I hope I am balanced.

Because the current on each leg of the feedline is the same.  If that is true, Then the feedline cant radiate.


We can drop the subject.  I dont think we are going to do anything but theorize. I was just trying to learn how changing feedline effected 40 meters.  I think walt hit the nail on the head with my situation.

C
Logged
Steve - WB3HUZ
Guest
« Reply #81 on: November 17, 2009, 03:31:03 PM »

Quote
It is the at current node where the maximum radiation takes place.

Only if the feedline is unbalanced. We have not determined that the line is unbalanced. My guess is that it is, but I don't see how changing the feedline would create unbalance where (apparently) none or very little existed previously, unless the geometry was changed.

Logged
k4kyv
Contributing Member
Don
Member

Offline Offline

Posts: 10062



« Reply #82 on: November 17, 2009, 04:07:29 PM »

The open wire fed dipole is best configured so that the feed point of the transmission line is near a current or voltage maximum point.  If it is midway between, it can be very difficult to tune.

My first experience with using the 80m dipole as a short dipole on 160 left me with a 3/8 wavelength wire, from the input end of the feedline to the end of the antenna.  I threw together a balanced link coupled tuner using a BC-610 split stator plate tuning capacitor (3/16" spacing, rated at 7kv breakdown).  I tried tapping down on the coil to get it to match the highly reactive load presented by the odd number of eighth wavelengths.  I finally found an adjustment that gave a 1:1 swr at the link when the coil was tuned to resonance, but I couldn't fully modulate over about 100 watts without the 7 kv tuning cap arcing over.

Finally, I added another 60' of open wire that I could switch in using a ceramic knife switch, bringing the total wire length to a half wave.  It loaded perfectly using parallel tuning, and I could run a full kilowatt 100% plate modulated without arcing over the capacitor.  The permanent tuner now uses a 300/300 pf 7 kv variable with a 50 pf 20 kv fixed vacuum capacitor bridged across it.  The coil is the balanced tank coil removed from an old Gates 250 watt transmitter that another ham had gutted.  The extra 60' of open wire is folded like a hairpin and strung between two metal fence posts about 30' apart and resembles a tennis net.
Logged

Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

- - -
This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #83 on: November 17, 2009, 05:51:05 PM »

added 2 feet of open wire line.  20 still wont tune lower then 2.0 to 1 SWR.  Feedline length seems to not effect things much. It did change the left cap knob on the KW matchbox.. from 10 to about 25.  Ideas?

C
Logged
KC2IFR
Guest
« Reply #84 on: November 17, 2009, 06:05:37 PM »

Started to scan the book. This project will take a while! Besides the time it will take, there will be a wide black line on the spine side of the page. This is because its a book......and I cant get it to lay flat on the scanner unless I pull the book apart page by page which I dont want to do. Even though the line is there, no text will be lost.

Bill

* page2.pdf (26.09 KB - downloaded 241 times.)
* cover.pdf (923.11 KB - downloaded 236 times.)
Logged
K5UJ
Contributing
Member
*
Offline Offline

Posts: 2845



WWW
« Reply #85 on: November 17, 2009, 06:26:47 PM »

Shane, Vertical radiation is not possible from my Open wire line. Thats been pointed out many times.  I think walt explained the gain on 40 meters.

You won’t know that for sure unless you walk outside with a field strength meter or a spectrum analyzer with a little loop or something connected to it and probe around.

Vertical radiation will increase local nearby signals on 40m, 75m and 160m during the day.

I don't see how you can walk around outside in the near field with some kind of FIM, even one of those cheap meters with a diode connected to a telescoping antenna, and tell the feedline is radiating when the whole area is swamped from RF coming from the antenna itself.  Maybe if you get the dipole way up away and tx with only a few watts but if it is only 50 feet up you'll get these readings but they'll be coming from the antenna and you'll think ur feedline is radiating it seems to me.

Rob
Logged

"Not taking crap or giving it is a pretty good lifestyle."--Frank
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #86 on: November 17, 2009, 06:29:03 PM »

Yeah..  I agree Rob.  If the system is balanced and you check the current on each leg, Its not radiating.

C
Logged
KC2IFR
Guest
« Reply #87 on: November 17, 2009, 06:34:50 PM »

BTW.......I think this was covered before on the board but I have found that Foxit Reader beats the Adobe pdf reader hands down for reading the pdf format.
http://www.foxitsoftware.com/pdf/reader/

Bill
Logged
K5UJ
Contributing
Member
*
Offline Offline

Posts: 2845



WWW
« Reply #88 on: November 17, 2009, 06:37:10 PM »

added 2 feet of open wire line.  20 still wont tune lower then 2.0 to 1 SWR.  Feedline length seems to not effect things much. It did change the left cap knob on the KW matchbox.. from 10 to about 25.  Ideas?

C
Clark, can't remember what your antenna is but on 20 I had trouble getting a match with my KW m/b feeding a 1/2 w. 20 m. dipole. On 14.340 only got as low as 1.9:1.  I added 12 feet of ladder line and that fixed it.   I had a 50 or 55 foot run and increased it to 62 or 67 feet so FWIW maybe you need to add more than 2 feet.   I inserted the extra length in my back yard and temporarily draped it around a 5 foot tall wood step ladder.  I guess I hunched that on 20 where a quarter wave is 16 feet, 12 would be enough to show a change but I got lucky and it was enough to match.  Throw another 10 or 15 feet in there. GL

Rob
K5UJ  
Logged

"Not taking crap or giving it is a pretty good lifestyle."--Frank
KC2IFR
Guest
« Reply #89 on: November 17, 2009, 07:21:11 PM »

Another Hmmmmmm,
How did this topic get so complicated......is it because the original post was bull or is it because some folks want to impress. I just cant figure it out Embarrassed
Logged
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #90 on: November 17, 2009, 07:37:48 PM »

Its an internet Forum. 

When you talk on the radio, do you talk about the same subject all night?  Smiley


I tried everything. This antenn wont tune 20 down flat. Oh well.

Clark
Logged
KC2IFR
Guest
« Reply #91 on: November 17, 2009, 07:50:58 PM »

If I start the QSO.......thats where it stays...... Roll Eyes

On the air...........not here......
Logged
Ed/KB1HYS
Contributing
Member
*
Offline Offline

Posts: 1853



« Reply #92 on: November 17, 2009, 08:21:07 PM »

Well, I guess I'll pour a little gasoline on this fire...

what about the mismatch at the antenna feed point??  Coax is 50 ohms normally, but a halfwave dipole is ~72 ohms so right there you've got a mismatch. Small so no problem right?

What about feeding that 72 ohm dipole with 300 (450, 600 etc) ladder line?  now you've got a BIG mismatch. So doesn't a lot of your power get reflected right back down the line? 

If it was to be max efficiency, wouldn't the feed line need to see an antenna with the same impedance?  like folding the dipole with the same spacing as the feed line to get nearly the same impedance???

Truth is, I don't know. My signal gets out, but this bit always made me wonder...
Logged

73 de Ed/KB1HYS
Happiness is Hot Tubes, Cold 807's, and warm room filling AM Sound.
 "I've spent three quarters of my life trying to figure out how to do a $50 job for $.50, the rest I spent trying to come up with the $0.50" - D. Gingery
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #93 on: November 17, 2009, 08:27:48 PM »

The open wire line is near lossless so the mismatch does not matter.  The only Draw back to the open wire line I can see is that its not easy to setup a true balanced system.  Its been alot of work and a real pain sometimes.

Clark
Logged
KC2IFR
Guest
« Reply #94 on: November 17, 2009, 09:28:12 PM »

Ed,
I think your question was covered a little earlier in this thread.......

Bill
Logged
KD6VXI
Contributing
Member
*
Offline Offline

Posts: 2045


Making AM GREAT Again!


« Reply #95 on: November 17, 2009, 09:32:08 PM »

added 2 feet of open wire line.  20 still wont tune lower then 2.0 to 1 SWR.  Feedline length seems to not effect things much. It did change the left cap knob on the KW matchbox.. from 10 to about 25.  Ideas?

C


Go from 6 inch spaced line to 4 inch spaced line Smiley  Make a transformer.  The difference in the line spacing will make a Z transformer, and you can fudge the spacing along with the MFJ to get the transformation ratio you want.

Still won't help with your vertical component.   Smiley

--Shane
Logged
WA1GFZ
Member

Offline Offline

Posts: 11152



« Reply #96 on: November 17, 2009, 10:15:09 PM »

Try adding 6 to 8 feet and I bet you will be fine. I used to run 2 Vee beams in parallel with one about 10 feet shorter than the other to lower the q that fixed everything. You are just outside the matchbox tuning range on 20m so it could take close to a quarter wavelength change to get you out of trouble. You can even hang the extra feed line off the match box with the end open. Just be careful as there can be some high voltage at the end. You could short the end but it will change things a lot on other bands.
Logged
KM1H
Contributing
Member
*
Offline Offline

Posts: 3519



« Reply #97 on: November 17, 2009, 10:16:10 PM »

Quote
The people here actually know something about Ham Radio and more


Aint that the truth. Compare that to the Antennex forum which thinks Cebik was a god. I never saw a bigger collection of fools and poseurs in my life and bailed out fast.

Tower Talk isnt much better and both are owner controlled so that only one side of an issue is tolerated.

Carl
Logged
KM1H
Contributing
Member
*
Offline Offline

Posts: 3519



« Reply #98 on: November 17, 2009, 10:39:41 PM »

Quote
What about feeding that 72 ohm dipole with 300 (450, 600 etc) ladder line?  now you've got a BIG mismatch. So doesn't a lot of your power get reflected right back down the line?

As mentioned the mismatch power loss will be low however the VSWR will still reflect back down the line. As already mentioned this may cause antenna tuning problems as well as radiation.  In the 40's Johnson sold the Q Feed System Q Bars which was simply a 1/4 wave 1/2" tubing transition from the OW line to the feed point. Its impedance is the geometric mean between the feedpoint and OW impedances. Old handbooks had construction details for 1/4" and 1/2" tubing and impedances at various spacings. Monoband of course but it allowed untuned feeders of any length.  I used the concept many years ago to feed a large fixed wire curtain array for 6M with almost 200' of OW and it worked great. At the house end I used a 1/4 wave shorted stub and a coax balun to the KW AM/SSB/CW amp. TVI and RFI all but disappeared.

Carl
Logged
K5UJ
Contributing
Member
*
Offline Offline

Posts: 2845



WWW
« Reply #99 on: November 17, 2009, 11:24:53 PM »

I don't see how you can walk around outside in the near field with some kind of FIM, even one of those cheap meters with a diode connected to a telescoping antenna, and tell the feedline is radiating when the whole area is swamped from RF coming from the antenna itself. Maybe if you get the dipole way up away and tx with only a few watts but if it is only 50 feet up you'll get these readings but they'll be coming from the antenna and you'll think ur feedline is radiating it seems to me.

No it will in fact work, but you have to do it at low power, no more than about 1 watt or so. I can even tell when the feedline is radiating with a cheapo radio shack FS meter. You just have to take some baseline readings first.

If you can get close to one of your wire elements then see how high the reading is. Decrease the power into the antenna to where the reading goes away when you move back a couple of feet, then measure/record that distance. I can do that from the roof.

Okay I'll have to give it a try.  Unfortunately I can't get close to the dipoles themselves--they're up too high but maybe I can still observe something.  I can tx a carrier on 40 m. with the dipole that's 130 feet long.  I think that will give me current out at the ends.  I can take a f.s. reading and back down the carrier until I barely see the meter move.  then go over to the feedline and take a reading about a foot away and see if there's any signal.  Something to do over the holiday.

Rob
Logged

"Not taking crap or giving it is a pretty good lifestyle."--Frank
Pages: 1 ... 3 [4] 5   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.071 seconds with 18 queries.