Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Link Antenna Coupler Circuit Setup




 
The AM Forum
April 13, 2021, 05:49:01 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: Link Antenna Coupler Circuit Setup  (Read 9997 times)
0 Members and 1 Guest are viewing this topic.
aa5wg
Member

Offline Offline

Posts: 430


« on: September 27, 2011, 09:28:16 AM »

My 13.5 quarter wavelength long (line plus 1/2 of dipole) 15 meter antenna system needs 600 ohms of capcitance reactance to neutralize 600 ohms of inductive reactance at a 45 degree (1/8th wavelength) point along the line at the tuner.  See below attachment.  This 45 degree point is not a low impedance point (current loop) or high impedance point (voltage loop) and thus is not a good spot for series or parallel tuning.

About 13pf of capacitance (600 ohm = 12.6pf at 21.0 mHz) will be needed to neutralize the 600 ohm inductive reactance.

(1) Is this 13pf of capacitance going to be the "normal" tank capacitor setting?  

(2) Or, is this 13pf of capacitance going to be an additional capacitor such as a single shunt capacitor (13pf) at the coupler and line connection point or two additional series capacitors (one 6.5pf in each leg of the line)  instead of the single shunt?

(3) Which tuning circuit would you choose, series or parallel and how should I set up this link antenna coupler circuit?
I prefer low impedance (series) tuning.

Chuck


* Photo_2011_9_21_13_30_1.jpg (1147.76 KB, 2550x3507 - viewed 589 times.)
Logged
k4kyv
Contributing Member
Don
Member

Offline Offline

Posts: 10062



« Reply #1 on: September 27, 2011, 12:25:02 PM »

If you use two series capacitors, one in each leg, I believe each one would need  to be 26 pf, since the two caps are in series, in order to get a total effective capacitance of 13 pf.  You could also try 13 pf across the line, although I am not positive how that will affect things compared to using the series caps. I would start out trying the series caps if I used series tuning and the parallel cap if I used parallel tuning, since the series or parallel capacitors could be combined with the resonating capacitance for the coil, in the same capacitor, although the 13 pf is only going to cancel out the feed line reactance and you will still need some capacitance to tune the coil. You will still be left with a resistive load somewhere between the high-Z needed for parallel tuning and the low-Z needed for series tuning. You might do better by tapping down on the coil to achieve the load.  That was my problem with my original attempt with the dipole on 160; I tried to tap down on the coil and still cancel the reactance with only the main parallel tuning cap across the entire coil, instead of using separate capacitor(s) to tune out the reactive component of the feed line load.

You should be able to use parallel tuning at the intermediate resistive load if you design the LC ratio of the main tuned circuit to work into  that lower impedance (less L and more C).  There are charts and formulae in the ARRL Handbook in the chapter "Connecting the transmitter to the feed line" that give the capacitance required to feed various loads, and probably similar  info in the Antenna Handbook as well.  I'm not sure about using series tuning for your higher intermediate impedance, but you might be able to find an L-C combination that would make that work.

Remember, with a parallel tuned circuit loaded down with a high resistance, the Q is increased by using less L and more C, while with a series tuned circuit working into a low resistance load, the Q is raised by using less C and more L.
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
aa5wg
Member

Offline Offline

Posts: 430


« Reply #2 on: September 28, 2011, 10:07:00 AM »

Don:
Should I be thinking the handbook capacitor and inductor recommendations, for the link coupler, are for ideal resistive points along the line?

And, the 600 ohms reactance found at the 45 degree point would be in addition to the above normal handbook recommended capacitive and inductive values?
Chuck



Logged
KA2DZT
Member

Offline Offline

Posts: 2173


« Reply #3 on: September 28, 2011, 01:07:30 PM »

I think that your calculations are affected by the length of your antenna for any given freq.  Also, the feed point resistance not being 600 ohms.  Whatever that resistance is, it will be the same every 1/2 wavelength down the line.  Any reactance on the line at the feed point will also repeat every 1/2 wavelength.  I think that the 1/8 wavelength point where the impedance is the nominal impedance of the line only works if the antenna is resonant and the line is feeding a 600 ohm load.

Just some thoughts, that may not even be right.  I'll have to do a little more research about this subject.

Sometimes it may better to just set up your antenna the best way you can and then just do some experimenting with the tuner.

Fred
Logged
aa5wg
Member

Offline Offline

Posts: 430


« Reply #4 on: September 28, 2011, 02:02:03 PM »

Fred:
Thank you for your thoughts.  I was wondering, at those 1/8th wavelength or 45 degree points along the line, if I could always use current feed if I  neutralized the reactance at the 45 degree points.  The neutralization would be in the form of either a shunt or series reactance.  This way I could use smaller variable capacitors in the link coupler.  

Maybe by doing it this way the voltage would be lowest at the coupler, current feed.

I am not for sure.  What do you think?  Any one else have a thought on this?

Chuck
Logged
KA2DZT
Member

Offline Offline

Posts: 2173


« Reply #5 on: September 28, 2011, 03:39:07 PM »

Chuck,

I think what I'm trying to say is that the 1/8 wavelength measurement may not start right at the end of the balance line due to mismatched impedance and an antenna that is not a 1/2 wavelength.  If your antenna is too short the balance line makes up that needed length to make it a 1/2 wavelength.  There probably is some ideal antenna length, short of a 1/2 wavelength, (for a given frequency) that will present a 600 ohm load right at the antenna center insulator.

When you deal with antennas, the only thing you know for sure is that at the far end of the wire there will be the highest impedance since the current is at it's lowest point and the voltage at the highest point.  Everything works back from the antenna ends.

So, I think whatever calculations you figure will quickly change when you change frequency or operate the same antenna set-up on a different band.  This is the reason you need wide ranging antenna tuners.

Once again I will reserve, that none of my comments may be correct.

Hopefully we can get more input from others.

Fred

Logged
Jim, W5JO
Member

Offline Offline

Posts: 2474


« Reply #6 on: September 28, 2011, 07:08:22 PM »

Chuck, you are too deep in the weed with this.  Calculate the maximum voltage node and current value for the power you will use and the minimum.  Then choose components that will reach the max and min. 

Simple as that.
Logged
aa5wg
Member

Offline Offline

Posts: 430


« Reply #7 on: September 28, 2011, 07:58:48 PM »

Hi Jim:
Calculate the max voltage and current is good. 

The 45 degree points or 1/8th wavelength points on the line at the tuner are a little tougher to match.  My 15 meter system is 1/8th wavelength to long for current feed.  I could add another 1/8th wavelength and easily voltage feed the antenna system.  But I don't want to add more feed line.  Extra line gets to be a mess.
 
I was interested in adding some capacitive reactance to shorten the antenna system length by 1/8 wavelength on 15 meters (21.0 mHz).

Chuck
Logged
k4kyv
Contributing Member
Don
Member

Offline Offline

Posts: 10062



« Reply #8 on: September 28, 2011, 09:33:49 PM »

Hi Jim:
Calculate the max voltage and current is good. 

The 45 degree points or 1/8th wavelength points on the line at the tuner are a little tougher to match.  My 15 meter system is 1/8th wavelength to long for current feed.  I could add another 1/8th wavelength and easily voltage feed the antenna system.  But I don't want to add more feed line.  Extra line gets to be a mess.
 
I was interested in adding some capacitive reactance to shorten the antenna system length by 1/8 wavelength on 15 meters (21.0 mHz).

Chuck

As your drawing shows, the tuner is seeing inductive reactance in that last 1/8 wavelength, so the capacitive reactance would cancel it out. But that won't take it back to a low-Z current loop.  The  resistive component will still be intermediate between what you see at a current loop and a voltage loop.

Would it be practical in your case to simply add 1/8 wavelength to each leg of the dipole? That would give you a total of 14 quarter-wavelengths and make the end of the feed line come out at a voltage loop and allow parallel feed with minimal reactance.  The 3/4 wavelength dipole would have slightly sharper lobes and deeper null off the end, but you would get a fraction of a dB gain broadside.
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
aa5wg
Member

Offline Offline

Posts: 430


« Reply #9 on: September 29, 2011, 09:12:18 PM »

Don:
I can't add any more feed line.  If I did I would have a spider web of transmission line on my hands.  

I found something in an old antenna handbook I need to look at.  Chapter 3 in the 1960 ARRL Antenna Book has on page 73 a section on INPUT IMPEDANCE and some info. on universal curves of resistance and reactance vs. line length for various Zr/Zo ratios and more.  I am looking for another ARRL Antenna book, maybe the previous edition, that had the same information but much larger graphs.

Let me read up on this and I will give you and others another shout on this post.

Chuck

I just found in the 1954 ARRL handbook, page 119, chapter 3 a section on "Coupling-Circuit Design".   And, in the 1964 ARRL Antenna Book, pages 91-95 a couple of sections that need to be reviewed.  They are Reactive Loads and Matching-Circuit Constants.

I was wondering, if I had a choice to load the ladder line with an inductive resistor at the end instead of the antenna, what resistor value should I choose to set up the coupler for a simulated default non reactive antenna?  Should the nonreactive resistor be 450 ohms or 600 ohms to match the Zo of the line or should I go with 75 ohms and add some reactance?  I don't know.  Then, I could leave the coupler set at the default setting for the 15 meter band and then apply shunt reactance at the line and coupler connection point.  I could then try capacitive or inductive reactance, if needed, to match the real antenna while trying to keep the coupler close to its "default" setting.

What do you think?

Chuck



Logged
k4kyv
Contributing Member
Don
Member

Offline Offline

Posts: 10062



« Reply #10 on: September 29, 2011, 09:40:18 PM »

Don:
I can't add any more feed line.  If I did I would have a spider web of transmission line on my hands. 

I didn't mean add any more feed line.  What about adding 1/8λ to each leg of the dipole and keeping the feedline length the same?
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
aa5wg
Member

Offline Offline

Posts: 430


« Reply #11 on: September 29, 2011, 09:45:38 PM »

Don:
I read antenna and was thinking line.  My mistake.  I cannot do that either.  At present the lot layout is to tight.  Check my reply #9 just above your last post.  I added some more info. to it.
Chuck
Logged
k4kyv
Contributing Member
Don
Member

Offline Offline

Posts: 10062



« Reply #12 on: September 29, 2011, 10:12:13 PM »


I was wondering, if I had a choice to load the ladder line with an inductive resistor at the end instead of the antenna, what resistor value should I choose to set up the coupler for a simulated default non reactive antenna?  Should the nonreactive resistor be 450 ohms or 600 ohms to match the Zo of the line or should I go with 75 ohms and add some reactance?  I don't know.  Then, I could leave the coupler set at the default setting for the 15 meter band and then apply shunt reactance at the line and coupler connection point.  I could then try capacitive or inductive reactance, if needed, to match the real antenna while trying to keep the coupler close to its "default" setting.

What do you think?

If the flat-top part of the antenna is close to a half-wave (1/4λ each leg), you would need a 75Ω non-inductive resistor, since the feedline at that point would be looking into approximately a 75Ω resistive load.  The 450Ω line would have about a 6:1 SWR, and the 600Ω line would run about 8:1, with the same impedance and reactance points along the line as the real antenna.  Your feed line is 12.5 quarter waves, and the flat-top adds another quarter wave to the total.

If you are talking about a resistor at the output of the tuner to replace and simulate the whole shebang, you would need a resistor plus enough inductance to produce 600 ohms of reactance.  I'm not sure off the top of my head without looking up the formulae (I never attempted to commit those formulae to memory since inevitably I get something ass-backwards) and calculatingt the resistive component, but it would vary, depending on the Zo of the open wire line, 450 or 600 ohms.
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
aa5wg
Member

Offline Offline

Posts: 430


« Reply #13 on: September 30, 2011, 03:12:55 AM »

I was thinking of connecting a non-inductive resister at the end of the line for this test.  I may have a 50 ohm, 400 ohm and/or a 600 ohm in the junk box.  I will have to look and see.  If needed I will get one with the right value.  Do you think this idea could work?  If so, what value for the non inductive resistor do you think I should use?

Chapter 3 page 119 under "Coupling-Circuit Design", from the 1954 ARRL Antenna Book, shows how to figure a rough calculation for the parallel impdance and reactance and series impedance and reactance.  The clincher is that these calculations are using a 70 ohm reference impedance for the antenna.  The resonant dipole (1/2 wavelength) at average height above ground.  My antenna is a 75 meter center fed doublet.  I don't recall what my antenna impedance is with an apex of 42 feet (inverted-vee).

I'll see if I can scan the 2 or 3 pages from the book.  It is interesting.  My antenna is using 450 ohm line.

In their example the antenna book says the series resistance is 120 ohms and the series reactance is 210 ohms, do you think they mean in one leg for the reactanc?  Their schematic, figure 3-77 c and d, shows one reactance element in series with the antenna resistance.  I usually keep every thing electrically and mechanically balanced (using two capacitors or two inductors if needed) by having one capacitor in series with each leg at the tuner.  I wonder if that means placing one capacitor with 210 ohms in one leg of the line at the coupler or one capacitor in each leg at 210 ohms, or one in each leg at 105 ohms or 420 ohms?  Don, do you know?   I was thinking they met these reactance numbers were in addition to the normal coupling inductance and capacitance valules for input and output sides of the coupler.  I will have to read the material again.

The lights just went out and then on again.  The rain storm must be getting worse.  I must shut down the computer.

Chuck
Logged
aa5wg
Member

Offline Offline

Posts: 430


« Reply #14 on: September 30, 2011, 07:05:00 PM »

This is very good reading.  (Attachments 1 and 2 of 6)
Chuck


* Photo_2011_9_30_23_7_6.jpg (1892.38 KB, 1700x2338 - viewed 542 times.)

* Photo_2011_9_30_23_9_58.jpg (1956.88 KB, 1700x2338 - viewed 506 times.)
Logged
aa5wg
Member

Offline Offline

Posts: 430


« Reply #15 on: September 30, 2011, 07:58:02 PM »

Attachments 3 and 4 of 6.
Chuck


* Photo_2011_9_30_23_12_32.jpg (2160.51 KB, 1700x2338 - viewed 486 times.)

* Photo_2011_9_30_23_14_49.jpg (2034.5 KB, 1700x2338 - viewed 536 times.)
Logged
aa5wg
Member

Offline Offline

Posts: 430


« Reply #16 on: September 30, 2011, 08:02:18 PM »

And, attachments 5 and 6 of 6.
Chuck


* Photo_2011_9_30_23_16_51.jpg (2286.87 KB, 1700x2338 - viewed 483 times.)

* Photo_2011_9_30_23_19_29.jpg (2535.39 KB, 1700x2338 - viewed 483 times.)
Logged
aa5wg
Member

Offline Offline

Posts: 430


« Reply #17 on: September 30, 2011, 08:22:20 PM »

test





Logged
KA2DZT
Member

Offline Offline

Posts: 2173


« Reply #18 on: September 30, 2011, 08:35:03 PM »

Impedance of a inverted V will be less than a straight dipole.  Should not be less than 90 deg.

Fred
Logged
aa5wg
Member

Offline Offline

Posts: 430


« Reply #19 on: September 30, 2011, 09:25:04 PM »

Thanks, Fred.
Chuck
Logged
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 0.088 seconds with 19 queries.