Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Adjusting Open Wire Length




 
The AM Forum
December 12, 2018, 09:25:58 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Adjusting Open Wire Length  (Read 1017 times)
0 Members and 1 Guest are viewing this topic.
Carl WA1KPD
Contributing
Member
*
Offline Offline

Posts: 1491



WWW
« on: November 11, 2018, 11:51:08 AM »

Good morning all,
Just got up a 240 ft dipole fed by138 feet of 600-ohm ladder line. That feeds into a Palstar BT1500A balanced tuner.
SWR, after tuning, is as follows:

1885   2.4:1
1945   4.9:1
   
3500   1.1:1
3600   1.0:1
3700   1.0:1
3800   1.0:1
3885   1.1:1
3900   1.1:1
4000   1.2:1
   
7030   1.0:1
7255   1.3:1
7290   1.3:1
   
14   1.1:1
14.1   1.0:1
14.2   1.0:1
14.3   1.0:1
28.5   1.3:1
29   1.6:1

I am not pleased with the 160 readings and it is my understanding that I need to trim the Ladder Line for best SWR.
My inclination is to shorten it based on the 160 readings but I am open to the experiences of others. Specifically, should I be adjusting by multiple feet or inches?

I am open to any other suggestions from hams with experience with this antenna
TIA
73
Carl
Logged

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
WA4WAX
Member

Offline Offline

Posts: 213


« Reply #1 on: November 11, 2018, 01:30:11 PM »

Dear Carl:

That feedline is about 1/4 wave on 160.....You are inverting 70 ohms to something around 4500 to 5K.  Tuna no like!  High RF voltage in the shack is bad Ju Ju IMHO.

Try 3/8 wave, or even 5/16.  Tuna will he happier.  Good luck.

Matt
Logged
WA4WAX
Member

Offline Offline

Posts: 213


« Reply #2 on: November 12, 2018, 07:51:18 AM »

.....One other thing: Try shortening the dipole to 230 feet as well.

Recall that the maximum Z transfer ratio of an L is Q^2 +1, so you see the trouble here.

Two back to back dipoles (1 wave) gives about 3 to 5K at the feed point.  1/2, 3/2, etc. will give around 70 ohms.

Print a Smith chart out.  Prime center will be 600 ohms, not 50.  Start with various loads and line lengths, and see if you can find a line length that will keep tuna happy on all bands.

Matt
Logged
WA4WAX
Member

Offline Offline

Posts: 213


« Reply #3 on: November 12, 2018, 06:45:03 PM »

Yet another idea:

Leave line and antenna alone.

Make a balanced low pass Pi or T for 160 meter service.  A receive type spilt stator will be fine on the input (Low Z) end.  Use 1/4 inch spacing on the output cap.  Use a matched pair of air core inductors,  with OD in the range of 2 to 3 inches.  Make each about 25 uH.  Find the tap points with a pair of gator leads, then install DX Engineering coil clips on the tap points.

A Pi will transform 4 or 5K to 50 ohms if desinged properly.

Use a MyAntennas dot com current balun or equivalnet on the input.  I think he has a good one for 160 meters.

M
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 6970



WWW
« Reply #4 on: November 13, 2018, 12:26:09 AM »

That Palstar is in some ways like a balanced PI, but.. maybe not enough range. Not speaking against the above advice at all but only to show the Palstar model details and the likely improvement of using a balanced pi network. (the top diagram is the cheap version but it's all 'hot') Totally agree on bigger inductances. Something to try, if you are using the whole inductance of the palstar might be to temporarily patch in 10uH extra in each leg and see if that helps. Usually the fail to tune, even in this weird situation, is due to something being maxed out or zeroed out.

Did you mean by a split stator to ground the 'center tap' or rotor shaft of the capacitors? There is debate on that. I certainly don't know much about that point at all.

Palstar said: The BT1500A 1500 watt balanced antenna tuner is the only 1500 Watt, Double-L Network antenna tuner on the market today covering 20m to 160m (2500+/-j2500) and 10m - 15m (1000+/-j1000). The BT1500A features two tandem mounted, precision ceramic roller inductors for a truly balanced tuner.

A dual section capacitor provides the correct amount of capacitance for both low band and high band operation, and is relay-switchable between the input and output side to provide the maximum possible range of impedance matching.


* balanced pi_.png (658.53 KB, 1369x1193 - viewed 92 times.)
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
WA4WAX
Member

Offline Offline

Posts: 213


« Reply #5 on: November 13, 2018, 11:43:12 AM »

Dear Patrick:

The bottom figure illustrates my idea perfectly.

Bonding the rotors to chassis gound is fine, but not neccessary if the balance is right on.

This may be verified by applying an RF probe to the floating rotors.  RF voltage should be miniscule, even with high power.

Here is yet another idea:

Home brew a BC-610 coil with bare #12 or #10.  Put about 15 uH on either side of neutral, and space the turns about 3/8 of an inch. Three inches in diameter would be fine.  Make sure it is symmetric!!  Space the two halves about 1.5 inches.

Put a 1/4 inch spaced or better spit stator Tx capacitor in parallel with coil.  You would want about 250 to 300 pF per scetion, and may need a fixed doorknob padding cap as well.  Dip to resonance to get an idea of the fixed cap needed.  Make sure this cap can take some serious RF current!

Make a link coil, and connect one end to the center conductor of a BNC, N, or SO-239 jack.  The shell of the jack goes to the chassis.  The oher end of the link goes to the stator a high C, low voltage rated (500 V fine) variable.  The rotor of this cap goes to chassis ground.  Feed with 50 ohm coax.  The reactance of the link on 160 m should be 50 ohms or close to it.  That will determine the diameter of the link, and the number of turns.  The series cap tunes the link, so no need for "swinging," etc.

Now, go out on the each half of the coil about 70% away from the neutral point, and tap with a large gator clip. Tap each in the same place.  Iteratively adjust parallel cap and series cap until you get a 50 ohm match.

If your cannot get a match, move the two gators in or out a turn or two.

This circuit will deal with a high Z load very well.

Good luck!
Logged
WA4WAX
Member

Offline Offline

Posts: 213


« Reply #6 on: November 13, 2018, 05:51:57 PM »

You may need 20 to 25 uH on either side of neutral, so check.

There is an article on BC-610 coil data in a late 40's or early 50's CQ Magazine someplace.  I suspect the Bunker of Doom guy has a file somewhere as well.

You would want to emulate the 160 meter coil.  If memory serves, 160 meter range BNC coils use formvar covered wire.  You will  want bare for this job.

M
Logged
WA4WAX
Member

Offline Offline

Posts: 213


« Reply #7 on: November 13, 2018, 07:04:59 PM »

Senior moment!

BC-610 coils, not BNC coils.

:-)
Logged
Carl WA1KPD
Contributing
Member
*
Offline Offline

Posts: 1491



WWW
« Reply #8 on: November 13, 2018, 07:29:32 PM »

Thanks. I'm going to try adding additional ladder line.
Having dropped mucho $$ on the Palstar, I'm inclined to go that direction before I muck around with the tuner
73
Carl
Logged

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
K6JEK
Contributing
Member
*
Offline Offline

Posts: 1121


RF in the shack


« Reply #9 on: November 24, 2018, 12:48:36 AM »

I'm pretty fond of the AA3RL Transmission line calculator. You could get a good idea of what changing the line length will do at various frequencies before you start splicing or trimming.  

It's an EXCEL spreadsheet. There are also website calculators.

https://www.qsl.net/aa3rl/tlcalc1.html
Logged
WA4WAX
Member

Offline Offline

Posts: 213


« Reply #10 on: November 24, 2018, 07:25:52 AM »

The BC-610 coil article may be found in the September 1949 issue of CQ.  Find a pdf on the VA3IUL website.

BTW, here is antenna to consider if you do not have a lot of space.  Yes, you will need a balanced tuner.

See the November 1952 issue of RCA Ham Tips.  You could scale it up for 160m, but it will be for short haul work on that band.

BTW, 30 uH on either side of neutral might be a bit better for a home made BC-610 style coil.


http://n4trb.com/AmateurRadio/RCA_Ham_Tips/rca_ham_tips.htm



Logged
Carl WA1KPD
Contributing
Member
*
Offline Offline

Posts: 1491



WWW
« Reply #11 on: November 27, 2018, 01:07:49 AM »

Thanks all,
I removed 10 ft from each leg and about 20 feet from the feedline. Behaving much better.
Appreciate all the input.
Carl
Logged

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
WA4WAX
Member

Offline Offline

Posts: 213


« Reply #12 on: November 27, 2018, 02:16:57 PM »

A little transmission line theory goes a long way.  :-)

Glad you managed to tame the beast.


Matt
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 6970



WWW
« Reply #13 on: November 29, 2018, 01:11:58 PM »

That was certainly a better fix than messing about with the matcher.
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
Pages: [1]   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 0.041 seconds with 18 queries.