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75 Meter Dipole: 10 Meter 7/2 Wave Doublet?




 
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Author Topic: 75 Meter Dipole: 10 Meter 7/2 Wave Doublet?  (Read 1489 times)
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AJ1G
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« on: February 13, 2023, 04:58:35 PM »

While poking around some bands I donít usually operate on from home last week,
noticed that while on my 75 meter coax fed dipole (fundamental freq about 3825 kHZ) into my IC7300, 10
meter signals were very loud.  Figured I would try working a few stations and mashed the 7300 internal antenna tuner button.  Was surprised when it found a match almost instantly, with a flat post tune up SWR over most of the lower half of the 10 meter band.  Did an SWR sweep
using the 7300 SWR function with the tuner out of the circuit and saw that the untuned SWR was
only 1.6 to 2 to one over 28-29 MHz, the plot indicated the minimum SWR was likely somewhere below 28.0. Doing the math I think the antenna is acting like a
7/2 wave extended dipole/doublet, which would likely have a low feed
point impedance.  However, I havenít been able to come across any specific feed point impedance data for a such a high order harmonic dipole.  I have seen some azimuthal plots which show some significant lobes at useful directions for the antenna that runs essentially east -west at about 45 feet.  The antenna seems to play really well, based on on-air (actual, not contest type) signal reports, especially CW CQ Reverse Beacon Network SNRs.

I do not have any antenna modeling software here, wondering if someone could run a 7/2 wave dipole model at 28 MHz and see what numbers fall out.  I am using a common mode choke consisting of about 25 feet of RG-8X on a 4 inch schedule 40 PVC pipe at the feed point.

I know that a 135 foot doublet fed with open wire feeders is the classic 80-10 meter multiband wire antenna, where one typically is running voltage feed and a high feed point impedance at whole multiples of the fundamental frequency for 40, 20, and 10 meters.  Again, Iím thinking my situation is actually operating at an odd harmonic, which is allowing it to work with a low impedance  coax feed.  The feed line is about 75 feet
of RG-8X between the transmitter and the common mode choke at the feedpoint.

Looking forward to feedback on any model results and also if others had any success in running a 75 meter coax fed dipole on 10 meters.





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Chris, AJ1G
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« Reply #1 on: February 13, 2023, 06:49:10 PM »

When I was a teenager,  I had a friend over to demonstrate ham radio.  I flipped the bandswitch of my Yaesu FR-101S to 10 meters and heard a 5H3 calling CQ.  So, I loaded up my Yaesu FL-101 and proceeded to call him.  I exchanged signal reports with Dave and had a brief chat with him.  My guest was impressed!  After the QSO was over, I proceeded to switch to 75 meters.  That's when I realized that, in my excitement, I was not on my tribander and was on the 75 meter inverted-V all along.

So, yes, a 75 meter antenna can be operated on the 7th harmonic.  It will be slightly out of band (as you discovered), but a minor modification as described in QST July 2002, pp. 32-34 will fix that without hindering your 75 meter operations.  See the link below if you are a member:

https://www.arrl.org/files/file/protected/Group/Members/Technology/tis/info/pdf/0207032.pdf
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73, Tony K4QE
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« Reply #2 on: February 13, 2023, 08:21:42 PM »

Years ago when i had a 75 meter dipole up i had used in on 10 a few times as well. Seem to recall that it worked best in directions that were maybe 20-30 degrees off from the ends of the wire lengthwise.
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« Reply #3 on: February 14, 2023, 04:24:59 AM »

Tony, thanks for the link to the 2002
QST article.  It identified the feed point impedance at 10 meters is about 120
ohms, and described use of a quarter wave 75 ohm matching section at the feed point for improving
the match to a 50 ohm feed line.  Iíll give that a try, will provide some improvement in performance and permit use of the antenna on 10 meters with other transmitters without an antenna tuner.


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Chris, AJ1G
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« Reply #4 on: February 14, 2023, 02:36:27 PM »

I installed the 75 ohm coax quarter wave matching section at the feed point as described in K8SYLís July 2002 QST article and it brought in a perfect 1:1 match on 10 even with the 7300 antenna
tuner out of the circuit.  Lesson learned - donít make a coil out of the 75 ohm matching section at the feed
point - totally messed up the match, worked perfectly just left hanging in a U below the feed point.

I did not need to add capacity hats to the antenna at about 8 feet from the feed point as described in the article, as my antenna was resonant sufficiently low enough on 75 already to make them unnecessary.  No observed effects on 75 meters from the added 75 ohm matching section.

Seems to be playing very well on 10, having trouble running CQs long enough to harvest Reverse Beacon Network SNRs because as soon as I start CQing Iím getting multiple stations calling me. Lots of
DX but few North America stations on apparently.

Thanks again Tony for the link to the QST article!

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Chris, AJ1G
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« Reply #5 on: February 14, 2023, 11:20:14 PM »

Glad I could help, Chris!
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73, Tony K4QE
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« Reply #6 on: February 15, 2023, 02:01:41 AM »

Hi, using 4nec2, modeled a dipole of #14 wire at 30 ft. Optimized it for resonance at 3880.
Switched to 28.5 MHz, and the swr and pattern images are shown below. Note the horizontal
pattern is taken at a vertical radiation angle of 15 degrees. The pattern shows gain over dBi (isotropic antenna). Note the main lobe is over 10 dBi.

The swr and pattern charts have been updated. At resonance in the 10 meter band the feedpoint impedance is 135 ohms.


* 75hor.png (83.95 KB, 950x983 - viewed 44 times.)
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km6sn
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« Reply #7 on: February 15, 2023, 02:04:13 AM »

swr curve - at resonance in 10 meter band feedpoint impedance is 135 ohms.


* 75swr.png (41.5 KB, 1600x973 - viewed 40 times.)
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« Reply #8 on: February 15, 2023, 02:14:10 PM »

Hi, using 4nec2, modeled a dipole of #14 wire at 30 ft. Optimized it for resonance at 3880.
Switched to 28.5 MHz, and the swr and pattern images are shown below. Note the horizontal
pattern is taken at a vertical radiation angle of 15 degrees. The pattern shows gain over dBi (isotropic antenna). Note the main lobe is over 10 dBi.

The swr and pattern charts have been updated. At resonance in the 10 meter band the feedpoint impedance is 135 ohms.
Chris is this rigged, looking this azimuth plot is this some kind of Valentineís spoof LOL
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km6sn
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« Reply #9 on: February 15, 2023, 05:30:03 PM »

Hi, using 4nec2, modeled a dipole of #14 wire at 30 ft. Optimized it for resonance at 3880.
Switched to 28.5 MHz, and the swr and pattern images are shown below. Note the horizontal
pattern is taken at a vertical radiation angle of 15 degrees. The pattern shows gain over dBi (isotropic antenna). Note the main lobe is over 10 dBi.

The swr and pattern charts have been updated. At resonance in the 10 meter band the feedpoint impedance is 135 ohms.
Chris is this rigged, looking this azimuth plot is this some kind of Valentineís spoof LOL


With all due respect, have you modeled the antenna? Upon what data are you basing your "spoof" comment? Please post your antenna  modeling data that contradicts what I posted. Thanks, Rod
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km6sn
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« Reply #10 on: February 15, 2023, 05:51:47 PM »

 Chris is this rigged, looking this azimuth plot is this some kind of Valentineís spoof LOL
[/quote]

Joseph,
better yet, the 4nec2 file is attached. Please change the file extension from .TXT to  .NEC

Please install 4nec2 on your computer. The download link  is
https://www.qsl.net/4nec2/

Email me at km6sn@fastmail.com; I  will send my phone number, and we can talk on the phone. I will guide  you  through the modeling process. It won't take long. 4nec2 is a derivative of the LLNL NEC program the government uses for all antenna modeling. I have reason to believe it  is fairly accurate.
Regards,
Rod

* 75.TXT (0.19 KB - downloaded 14 times.)
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AJ1G
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« Reply #11 on: February 15, 2023, 08:57:27 PM »

Rod thanks for all the effort you put into running the model!  Joe that is not a spoofÖI have come across similar azimuth plots for high order multilwavelength dipoles/doublets.  What Rod presented is typical.  Rod correct me if Iím wrong, but a classic ďall band 135 foot doubletĒ with tuned open wire feeders would have a similar pattern.  Whatís interesting about using a 7/2 wave doublet is that the feedpoint impedance is relatively low, the K8SYL QST article stated 120 ohms, and Rod modeled 135 ohms. And as demonstrated in the K8SYL article, addding a very simple matching device (in my case 7 feet 5 inches of RG-59 75 ohm coax) to a 75 meter dipole between the existing 50 ohm feed line and the feed point gives you a pretty decent 10 meter antenna with some useful gain in several directions.  In my case with the antenna running east west, the major lobes favor Europe, Southern Africa/Long Path Japan, Texas/Central America/Pacific, and NW USA/Short Path Japan.  And it has been working very well into all those locations!
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Chris, AJ1G
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« Reply #12 on: February 15, 2023, 10:09:36 PM »

Hi, using 4nec2, modeled a dipole of #14 wire at 30 ft. Optimized it for resonance at 3880.
Switched to 28.5 MHz, and the swr and pattern images are shown below. Note the horizontal
pattern is taken at a vertical radiation angle of 15 degrees. The pattern shows gain over dBi (isotropic antenna). Note the main lobe is over 10 dBi.

The swr and pattern charts have been updated. At resonance in the 10 meter band the feedpoint impedance is 135 ohms.
Chris is this rigged, looking this azimuth plot is this some kind of Valentineís spoof LOL


With all due respect, have you modeled the antenna? Upon what data are you basing your "spoof" comment? Please post your antenna  modeling data that contradicts what I posted. Thanks, Rod

Hi Rod & Chris,
Rod I'm not criticizing your antenna analysis data results at all, it looks great. My comment was about the two hearts on the azimuth plot, it was only a joke. My wife is right nobody understands my humor.  I'm sorry for the misunderstanding.
73s
Joe
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km6sn
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« Reply #13 on: February 15, 2023, 11:35:11 PM »

Joe,

Boy do I feel dumb! It nagged at me about the Valentine's Day reference.

NOW I get it!

Best Regards,

Rod
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« Reply #14 on: February 15, 2023, 11:51:40 PM »

Chris, yes, indeed the 135 ft CF wire have a similar pattern.

The antenna I modeled was a 122ft CF wire, length selected to resonate at 3880 kHz.

The feedpoint impedance varies somewhat with changes in height above ground
and ground conductivity.

Regards,

Rod
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« Reply #15 on: February 16, 2023, 09:11:33 AM »

I tried this with a 160 meter flat top before.

It was useless.

About 7 s units down from the vertical ground plane....  A 5/8 at 30 feet to the base, 50ish feet at the tip.

Maybe the 75 meter works better.

--Shane
WP2ASS / ex KD6VXI
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« Reply #16 on: February 16, 2023, 09:30:26 AM »

I tried this with a 160 meter flat top before.

It was useless.

About 7 s units down from the vertical ground plane....  A 5/8 at 30 feet to the base, 50ish feet at the tip.

Maybe the 75 meter works better.

--Shane
WP2ASS / ex KD6VXI

Think you are comparing apples and oranges - 160 meter flat top meaning a vertical tee? We're talking dipoles/doublets here.

I've run my 40 meter quarter wave vertical over 20 elevated radials as a 3/4 wave vertical on 15 with fairly good performance.  It does have a couple of strong  high angle lobes, but retains decent low angle response as well.
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Chris, AJ1G
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« Reply #17 on: February 16, 2023, 10:55:51 AM »

Hi Chris,

Now that the 75M dipole modeling has been done, I wonder if you have considered just a simple, coaxially fed,  1/2 wave dipole for 10M?

You might be able to cover both the ssb and AM portions of the band (swr)  -  and in real world use, get a broad, figure 8, predictable horizontal radiation pattern.   The vertical pattern will also be quite predictable at 25' to 60' off the ground.

I have a separate reference 1/2 wave rotary dipole for each band for 10, 15 and 20M and can say there are times when they fill in the pattern and perform better than the various fixed Yagis.    

Though I must admit, there are rare occasions on 10M/15/20M  when I switch to the 160M, 75M or 40M antennas and find a stray lobe that beats them all... pot luck.  :-)


T
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« Reply #18 on: February 16, 2023, 12:47:39 PM »

Actually Tom, I have been thinking of putting up a reference dipole for
some Antenna A/Antenna B comparisons.  Possibly a reference quarter
wave GP as well.

As a ďRun What Ya BrungĒ antenna it appears to be doing quite well.  It
does appear to be relatively deaf directly south, which is consistent with the model prediction.


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« Reply #19 on: February 16, 2023, 01:46:51 PM »

Chris,

Another idea is to put up a mono-band 10M dipole with two sets of legs.  One set resonant at 29 MHz (AM) and the other set resonant at 28.4 MHz (ssb).  I call this a "broadband fan mono-dipole," just like my 75M version with 3900 MHz and 3700 MHz legs.  SWR is < 1.5:1  across the whole 75M band.  That is unreal.  Something slightly akin to a log periodic

You need to spread the leg ends apart vertically maybe 5' or so to make it look like a bowtie.  (25' apart on 75M)   Wind up about 4 turns of coax at the feedpoint (5" diameter)  and add some 63 ferrite cores over the coax.  Beautius!

That will give you everything.... a great vertical and horizontal pattern, no feedline radiation and a good continuous match from the ssb to AM hangouts.  Drop the 28.4 MHz legs lower in freq if you want to cover the CW bands from 28 MHz up too.   Even three sets of legs are possible for a flatter swr across all. Experiment with leg length and end spread distance to find your favorite freq sweet spots.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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« Reply #20 on: February 17, 2023, 04:16:24 AM »

I tried this with a 160 meter flat top before.

It was useless.

About 7 s units down from the vertical ground plane....  A 5/8 at 30 feet to the base, 50ish feet at the tip.

Maybe the 75 meter works better.

--Shane
WP2ASS / ex KD6VXI

Think you are comparing apples and oranges - 160 meter flat top meaning a vertical tee? We're talking dipoles/doublets here.

I've run my 40 meter quarter wave vertical over 20 elevated radials as a 3/4 wave vertical on 15 with fairly good performance.  It does have a couple of strong  high angle lobes, but retains decent low angle response as well.


No.

My full sized 160 meter flat dipole.  Standard dipole, not configured as a T.

Sorry for the confusion.


--Shane
WP2ASS / ex KD6VXI
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« Reply #21 on: February 20, 2023, 07:51:52 PM »

Battle tested the 10 meter 7/2 wave coax fed doublet this past weekend in the ARRL DX CW Contest.  Performed very well running single op low power with the 7300 barefoot.  Played really well
to Europe, VK, ZL, KH6, KL7, and JA.  Ran strictly hunt and peck, on 10
meters to those locations, rarely didnít get through on first call even in traffic.  It did seem a bit
deaf and dumb to the Caribbean, but did excellent to South America.  10
was my most productive band. Ran dipole on 80 and 40, quarter wave wire ground
plane on 20, and used the 40 meter dipole as a Novice Special 3/2
wave on 15.  The Novice special also did very well on 15, usually got through on first call, especially to
JA, even though the incoming signals from there, most of which were from high power stations, were
uniformly weak and wispy with polar flutter.  Never heard so many JA, KH6, and KL7s in ARRL DX CW as this year.  Ended up with nearly 400 Qs in pretty much a part time vs. all out effort.  Majority of contacts were made on 10 and 15.

Going to make a quarter wave matching section from 75 ohm RG-59 for the Novice Special.  Came across some documentation on the subject that indicated the 3/2 wave dipole feed point impedance is also in the neighborhood of 120 ohms and can benefit from the quarter wave 75 ohm matching section treatment. Curious if it would work with my 40 meter quarter wave vertical run as a 3/4 wave vertical on 15.







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« Reply #22 on: March 05, 2023, 01:00:49 PM »

Successfully got a good match on 15 meters with an approximate 10 foot quarter wave matching section of RG-59 75 ohm coax at the 40 meter dipole feedpoint. Had to add some length to the 40 meter dipole to shift its fundamental resonance down below 7 MHz to get the match.  That is discussed in the ARRL Handbook, which cites diminished end effects of the 3/2 wave wire that result in its 15 meter resonant frequency being higher than 3 times the fundamental resonant frequency.  I can see the same effect on the 10 meter 7/2 wave operation of the 75 meter dipole.  The ARRL Handbook discussion deals with this issue by adding small wire ďcapacity hatsĒ about a third of the way out from the feed point.  I just am hanging short stub leads at the outer ends of the antenna, take them off when operating on the fundamental 80 or 40
meter frequencies of the antennas.  The Handbook discussion, interestingly enough, makes no mention of using the quarter wave 75 ohm matching section to improve the feed line matching, although it indirectly refers to the K8SYL 7/2 wave 10 meter operation QST article.


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