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




 
The AM Forum
December 12, 2019, 09:05:25 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]   Go Down
  Print  
Author Topic: Folded Unipole Antenna  (Read 631 times)
0 Members and 1 Guest are viewing this topic.
kc4mne
Member

Offline Offline

Posts: 13


« on: November 25, 2019, 10:54:56 PM »

Hello

Would like to add a "folded uipole", maybe "dual band shortened folded unipole" would be a better description to use on 80M and 160M. My tower is 70' tall. However have not found much on measurements online. Any ideas?  I have space for some ground radials, but not a perfect circle of radials around the tower.
Logged
W1ITT
Member

Offline Offline

Posts: 321


« Reply #1 on: November 26, 2019, 09:25:31 AM »

Get yourself a copy of Low Band DXing, by ON4UN, published by ARRL.  It's $45 but one of the best practical antenna books they have put out. In chapter 9, fifth edition , starting at about 9.62, John gets into the practicalities of just what you are attempting, and has interesting solutions for 160m as well as 80m.  If you have some good wide spaced or vacuum capacitors, the gamma and omega match configurations will allow you to do what you need to do.
For a ground system, consider using galvanized fence wire laid out on the ground.  Rob Sherwood at  www.sherweng.com  reprints an article he wrote years ago in Ham Radio Magazine with good measurements.  I use some screen under my 160m Tee, and there's a fellow on Ebay selling pvc coated galvanized chicken wire, 150 feet by one foot wide for $75.  I might add a bit of that to my own system for winter 160m use, then roll it up in the spring when I have to mow the lawn.
73 de Norm W1ITT
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1374


« Reply #2 on: November 26, 2019, 09:04:56 PM »

Below in PDF 1 is a typical "Folded UniPole" we use for broadcasting and it can be applied to almost any short antenna or any antenna less than 0.223 Lambda. The tower structure itself is fully grounded at the base to the radial field ring.

Lambda = 1 Wavelength.

Here is the problem in attempting to use it on two different frequencies: You will notice the item pointed out as "Impedance Matching Connection."

This is a short from the tower to the skirt at a specified point (height) in order to match the impedance at the specified frequency down at the feed-ring.

For 3.7 MHz, 1/4 Lambda is 67 feet so your tower is just over a quarter wave.

You could use a Slant Feed for 80m where a slant wire from your tuner output hits a point at the tower for about a 50 ohm impedance. See PDF 2.

Your tower is 0.135 Lambda at 1.9 MHz where at 1.9 MHz a quarter wave is 130  feet. A true folded unipole could be used but much experimentation and or computation would have to be done to find the "Impedance Matching Connection" point.
 

Phil - AC0OB

* Folded Unipole Broadcast Installation.pdf (36 KB - downloaded 56 times.)
* Slant wire Dawson_Slant_Wire_Shunt_Fed_IEEE_BTS_2010.pdf (895.36 KB - downloaded 38 times.)
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
kb2vxa
Member

Offline Offline

Posts: 141


I modulate, therefore AM


« Reply #3 on: November 28, 2019, 11:13:06 AM »

To keep it simple and the Greek alphabet out of it, use the standard design with 3 skirt wires spaced 18" from each tower leg and the upper commoning ring connected to the tower at the 60ft level. Don't forget to use Kevlar guys or steel broken into non-resonant lengths with porcelain egg insulators like you see on power poles and broadcast towers. If you use plain steel rope you'll wonder where the watts went. That's ideal for 160M and you'll find it has a low takeoff angle on 80-75M for great DX. Of course you'll need an ATU at the tower base, I'd use an automatic, weatherproof unit because remotely controlled units that were used on ship to shore shore stations are hard to find. That antenna can be used on 40M also but tuning gets a bit sharp, on 20M too sharp to be practical. You can use a broadcast engineer's trick to extend radials past structures on the property. Ring them with copper, solder the radials to it, and extend them on the far side to the edge of the property. They work best on the surface, but with copper thieves running around with cutting tools these days it's second best to bury them a couple of inches in slit trenches. You probably would shy away from the 120 radial system around AM broadcast towers, copper is way too expensive these days, but the more the merrier. Don't expect this to be a cheap antenna project, it isn't.

Take my word for it, the vertical folded unipole is fantastic on the low frequency bands having been adapted from AM broadcast use. I've seen a few used mostly to save expensive steel, the tower is a bit shorter than a series fed quarter wave AND it has a lower vertical radiation angle that increases a station's coverage watt for watt. K2PG Phil has them, one at his Pennsylvania QTH for his re-tuned AM broadcast transmitter, a 10KW Collins unit BTW, (;->) topped with a 2M vertical and center support for his dipole. I used the one at his West Creek, NJ QTH topped with an HF tri-band beam, 6M and 2M horizontal beams, the center support for a 160M open delta, and a 4 element 2M vertical folded dipole array. The tower may be used in the usual ways, side mount antennas become part of the tower and antennas above the top commoning ring are ignored by the RF when all coax lines come to the bottom and are grounded to the tower. 80M DX was absolutely fantastic, I had a 5 & 9 both ways QSO with England using only 100W PEP SSB while the dipole kilowatt guys pulled their hair out having no signal across the big pond.
Logged

73 de Warren KB2VXA
Station powered by atomic energy, operator powered by natural gas.
W1ITT
Member

Offline Offline

Posts: 321


« Reply #4 on: November 28, 2019, 12:22:18 PM »

Warren..    The modern automatic antenna matchers that I'm aware of are all Pi networks.  Like it or not, the Greek alphabet is going to be involved.    I suspect that the power ratings of the common ham radio ATUs  are optimistic at best, especially for AM use.  But if one has the extra drachmas to spend, there are some cool units out there.
73 de Norm W1ITT
Logged
kb2vxa
Member

Offline Offline

Posts: 141


I modulate, therefore AM


« Reply #5 on: December 01, 2019, 01:08:35 PM »

Hi Norm,

"The modern automatic antenna matchers that I'm aware of are all Pi networks.  Like it or not, the Greek alphabet is going to be involved."
Like Fred Flintstone (not flintstone mop) said, droll, very droll. On 14 March do you bake a Pi pie? On Einstein's birthday they virtually take over Princeton, NJ. BTW Raspberry Pi is not edible.

You're right and I wasn't covering all the bases, the automatic ATUs that go with 100W PEP mobile rice burners are meant to tune a 102" CB whip. Considering a typical AM/CW boat anchor puts out 100-140W carrier depending on the make, model, and band they'd hold up well under the 20% CW duty cycle, but hit one with 200-280W P-P AM and likely the magic smoke would waft skyward. Then there's today's elevated noise floor making a 600W output pretty much necessary to be heard well and forget it. One of my Frankenstein Laboratories BMF ATUs made out of 10KW AM transmitter parts would hold up well, but that would mean a trip to the dog house for every frequency change, most unpleasant in stormy weather.

I mentioned remotely controlled ATUs from defunct shore stations because Phil K2PG used one to tune his grounded vertical folded unipole in West Creek. His Pennsylvania unit is fixed tuned, a modified broadcast unit to handle the 10KW Collins 21E before the 480V 3 phase power pole rotted and fell, weather tight steel cabinet, ceramic tits and all. I remember the day when he blew up the West Creek one with a Collins 20V, bits of ceramic doorknobs all over the bottom of the cabinet. I'm no EE so I had to take my best guess, they were all 4.5KV caps and he had a bunch in the parts inventory, I used several smaller values to replace the larger ones thus distributing the circulating RF current and heat. Then out came the MFJ antenna analyzer and screwdriver to re-tune the whirring and clicking wee beastie.



* K2PG PA Cottage.jpg (294.74 KB, 640x480 - viewed 45 times.)
Logged

73 de Warren KB2VXA
Station powered by atomic energy, operator powered by natural gas.
W1ITT
Member

Offline Offline

Posts: 321


« Reply #6 on: December 01, 2019, 02:36:11 PM »

Warren..  Not necessarily just for use on March 14, but I do have a ceramic pie plate with the numerical value of pi worked out to a whole string of digits around the perimeter and a big pi symbol in the bottom of it.  It was given to me by a good friend who seemed to think I might have been a nerd at some point in my life.
Unless one is a QSY everywhere guy, most of us have just a couple different watering holes that we frequent on the bands, so a relay setup with a couple different tuning solutions might be handy,  I did that once for phone/cw switching on an 80m vertical.  I have one tuner that requires about a 500 foot hike out to the doghouse in the woods.  Here in Maine in winter, it's either snowshoes across three feet of snow or just live with that part of the band until spring.
I figure I can use the physical exercise, so I tend to avoid the store bought solutions, and I have plenty of Ls and Cs around so I brew my own tuners.  But a bulletproof 5kw autotuner would be a thing of beauty...  The government sites that I have seen all seem to favor broadband antennas and fixed matching.  Autotuning seems mostly to be on aircraft and ships.
73 de Norm W1ITT
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1374


« Reply #7 on: December 01, 2019, 04:45:01 PM »

Hello

Would like to add a "folded uipole", maybe "dual band shortened folded unipole" would be a better description to use on 80M and 160M. My tower is 70' tall. However have not found much on measurements online. Any ideas?  I have space for some ground radials, but not a perfect circle of radials around the tower.

Ok, here is the best I can do for now for a 70 ft. tall triangular metallic tower with a 1 ft. face for 1.9 MHz.

This was scaled from one of our broadcast towers.

The impedance matching point is calculated to be about 33 feet above ground with an 18 inch spacing of the three #12 insulated wires from the tower. The impedance matching point here also serves as the "spreader" to keep the wires parallel along the length of the face of the tower.

You may have to experiment as to the exact shunt impedance matching point height and before you affix cross (spreader) arms at that point.

The feed-point HOOP needs to have insulated cables with insulated spacers and a turnbuckle below it in order to tension it and to keep the 3 skirt wires taught. The three tensioner cables need to be spaced 120 degrees apart.

The diagram shows the dimensions.


Phil - AC0OB

 

* Folded Unipole for ARS.pdf (133.68 KB - downloaded 30 times.)
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
Jim, W5JO
Member

Online Online

Posts: 2451


« Reply #8 on: December 01, 2019, 08:04:16 PM »

As I recall the MFJ-998RT, and maybe the LDG 1000, remote tuners are L network designs.  I read that the MFJ is but not the LDG. 

The MFJ has an output for coax or single wire.
Logged
kc4mne
Member

Offline Offline

Posts: 13


« Reply #9 on: December 09, 2019, 10:14:59 PM »

Thanks for all the ideas
AC0OB, what would the dimensions be for the 80 M version. Also the 70' tower is close to a building so the skirt wires can only be 12" away from the corners of the tower. Tuner not be a problem, will be L fixed on one frequency with vac cap and big coil. no store bought garbage. Transmitter on 160 will be one of my 1kw broadcast boxes, 80 will be 1kw or Collins 21E.  Ideal setup would be same ant with relays as needed to match to 160 or 80 with different feed lines and tuner, only common antenna.

KC4MNE


* madmanatwork.jpg.jpg (269.83 KB, 1632x1224 - viewed 22 times.)
Logged
kc4mne
Member

Offline Offline

Posts: 13


« Reply #10 on: December 09, 2019, 10:16:50 PM »

I also have a second 50' tower on the site that is not next to building. all towers are self supporting crank ups, no guy wires.

KC4MNE
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1374


« Reply #11 on: December 10, 2019, 01:41:40 AM »

Thanks for all the ideas
AC0OB, what would the dimensions be for the 80 M version. Also the 70' tower is close to a building so the skirt wires can only be 12" away from the corners of the tower. Tuner not be a problem, will be L fixed on one frequency with vac cap and big coil. no store bought garbage. Transmitter on 160 will be one of my 1kw broadcast boxes, 80 will be 1kw or Collins 21E.  Ideal setup would be same ant with relays as needed to match to 160 or 80 with different feed lines and tuner, only common antenna.

KC4MNE


Okay so I think you're saying a 160 meter folded unipole is out.

As I stated earlier if your tower is well grounded and has a radial field, and for 80m, run a wire from your tuner to a spot on the tower at about a 45 degree angle.

* Shunt fed Monople.pdf (91.22 KB - downloaded 16 times.)
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
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 0.064 seconds with 19 queries.