160 Meter Transmission Line Study - SWR and Losses

[Tom WA3KLR]

I did a study on driving a 75 meter dipole on 160 meters.  The goal was to see what the range of feed point SWR is, just for curiosity sake.  I used the EZNEC 3.0 antenna program for obtaining the feed point impedance numbers (called Source Data).  I had an Excel r.f. utility calculator I made previously, for computing SWR from inputting impedance in rectangular coordinates and the system impedance, and then I used the W9CF transmission line program, recommended by someone recently here on the AM Forum, to get the transmission line losses.

The dipole model used was #14 wire over real ground at 40 foot height.  The antenna was resonant very close to 3880 with a feed point impedance of 63.9 Ohms + j1.11 Ohms at 3880 kHz.

The transmission line length used was 125 feet.  I have displayed the matched line loss and velocity factors for the ladder line families that W9CF used in his program.

Here are the findings:

Frequency                                 1.8 MHz.         1.9 MHz.             2.0 MHz.
Feed point Impedance              8.71 – j1274    9.63 – j1175    10.66 – j1084 Ohms
Feedpoint TL Mismatch SWR
      51 Ohms system                   3662:1              2817:1             2166:1
    300 Ohms system                     656:1                509:1               396:1
    450 Ohms system                     466:1                365:1               287:1
    600 Ohms system                     380:1                301:1               240:1
  1275 Ohms system                     293:1

125 feet TL loss, dB.
RG-58A                                          25.4                 24.4                 23.3
RG-8X                                            23.6                 22.7                 21.8
Belden 9913                                   20.3                 19.4                 18.5
300 Ohm  Tubular  0.12/0.80           12.1                 11.3                  10.4
450 Ohm Window  0.09/0.95            9.6                   8.9                   8.2
600 Ohm  Ladder   0.02/0.97            4.4                   3.9                   3.5
              (loss db/100ft./vel. fac.)

* All of the dB losses corrected 12/18/06.  Ladder line losses have increased as much as 3 dB., and coax losses have minor increases from previous version.

Due to the transmission line losses, the SWR at the transmitter end of the line is lower than the numbers stated in “Feedpoint TL Mismatch SWR”.

As you can see, the SWR on the feed line is extremely high, requiring the use of very low loss transmission line for practical use (which we already knew).  You are feeding an antenna whose feed point impedance is no where near the transmission line impedance.  You would need a transmission line impedance approximately equal to the feed point reactance for the closest match, and even then the SWR would still be high.

[AB2EZ]

Tom

Another conclusion one can draw from your calculations is the following:

Even with an ideal (zero loss) transmission line, and even with an ideal (zero loss) matching network/tuner... the radiation resistance is only ~ 10 ohms. What this means (another way of saying the same thing) is that most of the power that "radiates" from the antenna will actually couple into non-propagating/ "evanescent" modes... which end up generating heat when they produce currents in the imperfect ground and in nearby conducting objects. Some people will recognize these non-propagating/evanescent modes as being associated with the "near field".

Thus, even under ideal circumstances, basic physics principles imply that (roughly) for every factor of 2 that the dipole antenna is short of a half wavelength, you will lose 6dB of propagating power (i.e, stuff that actualy propagates out as a travelling wave), even if you use perfect ladder line and a perfect tuner/martching network.

Best regards
Stu

[WA1GFZ]

Interesting information even good 600 ohm line you lose almost 2 dB.
I wonder what happens with other antenna configurations. How about a shall loop?

[WA1GFZ]

Stu,
My brain was bumming at a 3 dB number..

[Tom WA3KLR]

Yes, using an electrically short antenna is a killer (electronically) for many reasons.  The resistance I show is the “real”, total feed point resistance – this is not the radiation resistance.  (I am using a simulator program.)

I went back to EZNEC and made the ground PERFECT and the wire loss = zero.  Then the resistive part of the feed point impedance displayed is the radiation resistance.  It was 2.14 Ohms.  This is a radiation efficiency of about 26 %.  The 8.36 Ohms feed point resistance I got for copper wire with real ground is the total resistance which is the result of the wire and ground losses included.  In real life you have the adverse effect of surrounding objects – trees and buildings to lower the radiation efficiency further yet.

So in real life you have the reduced radiation efficiency of the antenna plus the feed line losses plus the antenna tuner losses.  A high half-wave dipole helps to overcome much of these losses.  A resonant half-wave dipole fed with 125 feet of RG-8X would have 0.44 dB of feed line loss, no tuner losses, and a little higher radiation efficiency than driving the 75 meter dipole.

I just thought it would be neat to come up with and "know"  theoretical SWR numbers at the ladder line of the 75 meter dipole fed at 160 meters.  Even if we could put ourselves at the feedpoint invisibly, we still do not have hand-held instruments that handle the combination of feed-point impedances and transmission line impedances involved.

[WA1GFZ]

so maybe it is better to drop vertical wires off the end to get all the length possible
if you only have room for a short antenna.

[Steve - WB3HUZ]

Yep. Almost any way you can add wire to the system can help. That said, at least two forum members are running straight up 75 meter dipoles on 160 with excellent results.

[WA1GFZ]

Yes Steve, but 2 dB is a lot of power using the best feedline.
Ape man told me to stick with the short dipole so it works better on 40
but now I am leaning back to the vertical sections of wire to reduce losses.
I was even looking at the center conductor of a dead section of 7/8 Andrews
looks like thin wall 1/4 inch tubing. Might be too heavy but one could dream.

Tom,
what do you think about a 150 foot dipole with 50 foot vertical ends under the same test conditions total lenght 250 feet.. Guess you need to throw in some ground loss to be fair.

[W2VW]

Frank, there's a big difference when you get up to 150 or 160 feet center fed from what you get with 125' center fed. When you start dropping wire off the ends you could land up with a dummy load on the higher bands. Plot the current points to see max radiation points. I've put (3) 160 foot center fed dipoles into service in the last year and am going to install a fourth for myself. They all work fine from 160 up to 40 with a broadside pattern.
    At 125' any extra wire is a big help.

[WA1GFZ]

Dave higher bands I could do a 40 meter dipole across the top of the lot facing EW that would have a 100 ft average hight above the salt marsh 200 feet west.
Higher bands will be no problem.

[W2VW]

Yep. Almost any way you can add wire to the system can help. That said, at least two forum members are running straight up 75 meter dipoles on 160 with excellent results.

Yes, but they won't work on ssb. They only work on CW and AM.
You need a carrier in order to obtain the conjugate match. This is a scientific fact.
It's a good thing 'cause all those guys who can't hear each other right now on 75 might come down.

[w3jn]

Yep. Almost any way you can add wire to the system can help. That said, at least two forum members are running straight up 75 meter dipoles on 160 with excellent results.

I don't know how excellent my results are, but it's more than adequate to get on the top band.  75M dipole, 16 ga open wire line, and homebrew tooner.

[WA1GFZ]

John,
1 how high is the ant
2 how long is the feedline
3 what is your tuner L and C at match. I saw the tuner schematic
so only interested in the amount of C you need to get a match.

You do put a good signal up here

[Steve - WB3HUZ]

No matter what you put up on 160 meters, unless it's WAY high, ground loss will come into play. There's nothing you can do about it, so put up the antrenna system that will be the most efficient in and of it's self, and rock on.

[Tom WA3KLR]

Frank,

Since you have 50 foot danglers on the ends, I assume your antenna is 65 feet high.  The antenna you describe is a resonant half-wave on 1921 kHz.  This is very good.  The average gain of this antenna is about 2 ½ dB better than the 75 meter-sized dipole I analyzed at 40 feet.  The feed point impedance is in the vicinity of 50 Ohms and does not get to the 89 – 90 degree phase angles of the electrically short 75 meter dipole.

The transmission line losses for feeding your antenna

Frequency                                 1.8 MHz.         1.9 MHz.             2.0 MHz.
Feed point Impedance           36.2 – j121       43.8 – j21           53.0 + j79.5 Ohms
Feedpoint TL Mismatch SWR
      51 Ohms system                    9.95:1                1.6:1                  4.1:1
    300 Ohms system                    9.65:1                6.9:1                  6.1:1
    450 Ohms system                    13.3:1               10.3:1                 8.8:1
    600 Ohms system                    17.3:1               13.7:1               11.5:1
 
125 feet TL loss, dB.
RG-58A                                        3.1                 0.8                 1.33
RG-8X                                          2.5                 0.6                  0.82
Belden 9913                                 1.4                 0.3                  0.37
300 Ohm  Tubular  0.12/0.80         0.85               0.5                  0.31
450 Ohm Window  0.09/0.95         0.9                 0.6                  0.37
600 Ohm  Ladder   0.02/0.97         0.3                 0.2                  0.13
              (loss db/100ft./vel. fac.)

* The dB losses for 1.8 and 1.9 MHz. corrected 12/18/06.  Ladder line and coax losses have minor increases from previous version.

One interesting thing here is that among the ladder lines, the 450 Ohm ladder line did worse than the 300 and 600 Ohm ladder line.  This is due to the opposing factors of loss parameters given and the fact that the SWR is lower for the 300 Ohm line.  Even though the 600 Ohm line has the highest SWR, it has the lowest loss and therefore the lowest net loss of all the transmission lines.

Even the RG-58A and RG-8X are useable in this situation; the maximum r.f. voltage on the 50 Ohm coax would be less that 800 Volts with 1500 Watts, occurring 25 feet in from the antenna feed end.

At 1.8 MHz. however, the 600 Ohm line with 1500 Watts has just under 4000 V. rms at the transmitter end of the line.

[w3jn]

My 75 M ant is actually a fan dipole.  Each leg has 2 wires separated at the ends by a foot or two.  It's up about 30-35 feet.

Tom, thanks for this analysis.  Does the fan dipole configuration help at all?  My goal when building it was to broaden out its response somewhat on 75, but I didn't really think about 160 back then.

[WA1GFZ]

Tom,
Very good information. Now back to John's question. I've had good luck with Fan Vee and Rhombics when I was a kid. It made it a lot easier to load my match box over a broader range.
I could also do a fan spread as much as 20 feet. How about a pair of vertical drops at each end.
Maybe tie the ends to a common insulator at the ground to reduce coupling to ground with 2 ends apart. My case I will be on the side of a hill sloping west. The two tree supports are a good 70 to 80 feet high. 
How about John would he gain anything adding 40 feet vertical??

[K1JJ]

Precise and detailed computer generated info, Tom.

Sure beats old ham wive's tales and anecdotal antenna stories of what worked for Uncle Joe in the 50's...

Keep up the interesting posts!

73,
T

[AB2EZ]

Tom (KLR)

I fully agree with Tom's (JJ's) observation about the quality and usefulness of your work on this.

Stu

[k4kyv]

One interesting thing here is that among the ladder lines, the 450 Ohm ladder line did worse than the 300 and 600 Ohm ladder line.  This is due to the opposing factors of loss parameters given and the fact that the SWR is lower for the 300 Ohm line.  Even though the 600 Ohm line has the highest SWR, it has the lowest loss and therefore the lowest net loss of all the transmission lines.

I'm not sure how much line impedance has to do with it.  The tubular stuff, and the "window" ladder line have more dielectric material between the wires to cause dielectric losses. It would be interesting to see how the figures would compare if each line were made of bare wire spaced apart with lo-loss spreaders such as ceramic.  300-ohm line would be more easily constructed as multi-conductor open wire line, than with just two-wire line.  300-ohm solid flat TV ribbon has about as much loss to it as coax, but I have seen it used (or misused?) to make tuned feeders up to the KW level.

I used to have pretty good luck with open wire TV lead-in, the stuff made of #18 copperweld wires spaced about an inch apart with plastic spreaders spaced about every 6."  But I prefer homemade line using ceramic spreaders.  These used to be dirt cheap at hamfests, but now, like tubes and audio transformers, the prices are usually sky-high.  (Audiophools have "discovered" a bogus use for them?  )  Plexiglas rods, and strips cut from plexiglas sheets work well, are immune to UV damage, and are less fragile than ceramic spacers but have slightly more dielectric loss than good ceramic insulators.

The simplest example of multi-wire open wire line is 4-conductor, laid out in a square pattern, with diagonal corners strapped together.  I have seen diagrams of line with as many as 16 conductors, along with formulae for calculating the impedance.  This allows for (relatively) low impedance line without ridiculously close spacing and/or large diameter conductors.

Does anyone know of a readily available source of this data, particularly on line?

[Tom WA3KLR]

John’s 75 fan dipole on 160:
                                                      1.8 MHz.            1.9 MHz.              2.0 MHz.
75 Meter 2 wire dipole –          9.39 - j1269     10.27 - j1171      11.22 - j1081
 - 33 feet above ground

75 Meter 4 wire dipole –           6.31 - j604         6.91 - j555           7.57 - j510
 - 33 feet above ground
(wires 2 ft. apart at ends )

The fan dipole’s feed point impedance is lower and the reactive component is less.

EZNEC radiation pattern gain shows 1.7 dB more gain for the fan dipole surprisingly!?

I’ll assume John is using 600 Ohm ladder line:
                                                           1.8 MHz.           1.9 MHz.           2.0 MHz.       
600 Ohm line losses in my first post             4.4                  3.9                     3.5
- With John’s fan dipole feed impedances     3.0                  2.7                     2.4
                                          Difference   - 1.4 dB.           - 1.2 dB              - 1.1 dB.

* All of the dB losses corrected 12/18/06.  Ladder line losses have increased from previous version.  The Differences all increased by -0.3 dB.

So John in this example you may have about 0.9 dB less loss in the ladder line due to your fan dipole impedances.  (Your line may not be 125 feet though.)  Along with the EZNEC’s  + 1.7 dB gain? that’s +2.6 dB for you on 160 with the added wires, if you can believe all of this.

That’s all of the simulations for today folks.  Get your own copy of EZNEC (some bucks) and the W9CF tx line program (free).

As Steve and Stu have already said, if you can make an electrically short antenna longer it is going to be a help.  The ratio of radiation resistance to total feed point impedance improves and the transmission line losses will probably lower too.

Don,

The total losses on the ladder lines, due to the the extremely high SWRs is multiplied greatly over the basic "matched" loss of the lines usually expressed in dB/100 ft.  The various constructions you are mentioning have a great effect on the losses.  We are almost comparing apples and oranges in the 3 ladder lines used in the W9CF program. If you made your own lines and all very similar except for the spacing, I would think the losses/100 ft. are about the same.  The net result comes down to which line impedance has the lower SWR on it for the application and that one will have the lower net loss.

Once it appears that a low-loss low impedance line is the ultimate answer then yes you may be looking at open wire lines that need 4 or more lines in two facing planes for a low Z line.  I don't know of source on this off-hand, except some handbooks show formulas for a variety of transmission lines and computing the resulting impedance.  As you say, I don't think you need heavy wire with close spacing, just many wires, a rat's nest.

* Tom and Stu,

Thanks for the comments, much appreciated.

By the way, the HUZ-man has a terrific signal up here on 160.  What does he use for an antenna?

[K1JJ]

BTW, Tom...

Your 6M matching design is completed and working FB.

If you remember, I'm matching the phasing harness of an eight, 4el 6M Yagi stack  (20ohms) into a 75 ohm hardline.  I made two separate matching units into one waterproof box for two arrays back to back. After some tweaking I was able to get a 1:1 match into the hardline. The final L/C values were very close to your estimates.

Both arrays are working and I've found them to have about about 10db gain over a single 3el Yagi reference antenna at 190'. So, the stacks are working textbook.

Your other project, the cathode audio drivers, are built but need testing. I'll let ya know how they work out.

TNX for the help, OM.

73,
T

[K1JJ]

Speaking of ultra close spaced open wire line...

Last year I planned on using open line to feed my 6M Yagi stack. I bought ten feet of 3/8" diameter Teflon rod and 600' of #12 solid enamel insulated copper wire.

I made small Teflon spacers that were  5/8" long with holes on the ends for the wire. The open wire was spaced 3/8" and had a spacer every 3" apart.  I think the impedance was down around 200 ohms or so?

Anyway, that was a simple way to get lower impedance line. I opted out for coax for the project to keep it environmentally stable, and now glad I did. Maybe I'll use this openwire for something, someday.

So, how low is the impedance of this 4 wire type design?

T

 

[Steve - WB3HUZ]

EZNEC radiation pattern gain shows 1.7 dB more gain for the fan dipole surprisingly!?

Beaware of the gain figures given by any NEC based modeling program for antenna close to the ground wrt wavelength. The numbers are usually high.

[w3jn]

Very cool, Tom, thanks a million for the sim.  That antenna works very well on all bands despite its low height.  It's a bit funky tuning up on 40 but works there despite that.