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[Articles Home]  [Add Article]  

Feeding an LPDA Beam Antenna

Steve Katz (WB2WIK) on June 22, 2014
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Feeding an LPDA Beam Antenna

I kind of like log periodic rotatable beam antennas. For the HF bands, I’ve had monobanders, tribanders, 5-banders and even 6-bander designs using traps, linear loading, mutual coupling and all sorts of stuff and everything’s “worked,” but some antennas are more difficult to install, or adjust, or maintain than others. They all have their good and bad points: They’re great when all tuned up and working fine, and some are painful to “keep” up and working fine, over time.

When I was in my early 20s I had occasion to visit and even operate from the U.S. Army amateur radio station K2USA at Ft. Monmouth, NJ. The “station” was very good, with operating suites in beautiful wood-paneled rooms – unlike anywhere I’d ever operated before. And there were lots of antennas for MF through microwaves. One antenna I recall admiring was this large LPDA rotary beam which covered most of the HF spectrum. I’d never been close enough to touch it or work on it, but it was impressive and looked “big and heavy.” I figured I’d never have anything like that, and I never really have.

Many years later, having moved into my 14th home, this time in southern California, I went through my normal routine of unpacking and then figuring out where to put a tower. It only took a few minutes to decide where the tower just had to go, on our small city lot: It had to go right behind the house, west of the swimming pool, to allow it to tilt up into position without interference from stuff that I couldn’t easily move, like the house, trees, utility wiring and the pool equipment (filter, heater, etc). So, that’s where the hole was dug and the concrete poured as a tower foundation.

I wanted an HF beam to cover 20-17-15-12-10 meters if possible, all in one antenna. Various designs can do that, but I kept thinking of that big Army LPDA and how well it worked. After a bit of research it seemed one equivalent to what they had might cost about $20,000. There had to be alternatives.

Stumbled across a company called Tennadyne, which was in Texas at the time, and selected a 6-element, short-boom LPDA that only cost about $500 or so at the time. Much better. I had no experience with this antenna at all, but ordered one (in early 2001) and assembled it on the roof of the house so once built it would be closer to its final resting place atop the new tower, which was only a foot behind the house.

The manufacturer recommended using a coaxial RF choke current balun, not supplied but easy to build. They also recommended installing said balun at the very front of the antenna close to the antenna connections, and routing the coax down the lower of two booms (LPDAs often have two booms, which support and interconnect the elements and act as a transmission line) back towards the supporting mast, where a rotator loop would be formed, and then the cable routed down a tower leg.

So, I installed it exactly that way. It worked, and seemed to work fine, but I had nothing to compare it to at the time. VSWR across each of the five covered ham bands was pretty good, and the antenna had a clean, directional pattern so it must have some gain. There’s no place in an LPDA to create any “loss,” as it’s an all-aluminum antenna with no coils or traps or anything. Whatever reaches the antenna must radiate somewhere, so finding a clean, directional pattern should be a good indication it has some gain towards the peak direction.

The VSWR curves were a bit “bumpy,” but no VSWR above 2.0:1 was observed anywhere except at the extreme high end of 10m, above 29 MHz. Not bad.

Then, I had occasion to visit a local mountaintop where there are several commercial/government communications facilities, including some using LPDAs. In this case, the LPDAs were obviously for higher frequencies, above 50 MHz, as the elements were too short for HF; they were also all fixed-aimed and not rotatable, and appeared very heavy duty – which made sense, since this was atop an 8000’ hill that can get some nasty winds and even the occasional snow/icefall despite being only about 75 miles outside Los Angeles.

I observed something about those “commercial” LPDAs, and that was they all used split booms and none of them had their coaxial transmission lines routed along the lower boom; they all had the line “fall” from the feedpoints down a few feet below the antenna, then pulled back towards their supports in a big, hanging “loop” (or really, half-loop). I wondered if there was a reason for that.

It was about time for me to replace my coaxial choke current balun, as I could see with binoculars that the tie-wraps I had used to secure it to the end of the lower boom were breaking (a few years of UV exposure did them in); I figured if I’m going to re-secure the balun, might as well replace it at the same time. So, I wound a new choke (7 turns of coax in about a 5” diameter coil, secured to hold its form) and cranked down the tower.

This time, instead of tie-wraps which I knew would fail, I used just double-braided Dacron 3/32” rope to secure the balun to the lower boom, and instead of routing the cable down the boom, I let it “hang” from the antenna to form a big half-loop like I’d seen on the commercial installations up on Frazier Peak. Wonder if that will make any difference?

Yes, it did. Although I have no way to measure gain of HF antennas, I can measure SWR. The SWR curves across each of the ham bands is less “bumpy” and now smoother, and nowhere exceeds 1.5:1 except above 29 MHz, where it reaches about 1.7:1 maximum. The only thing I changed was the way the coax was routed – even the balun, although “new,” was made the same way as the first one, from the same coax. (The coax used for the choke is a UV-protected version of RG8X, called IEWC #9092-IIA. It’s still just RG8X, but the outer jacket material is non-contaminating polyethylene instead of the typical contaminating type PVC used on most RG8X. The reason to use this stuff, for me, was purely “longetivity” with UV exposure.) My “main” transmission line which goes from the rotator loop down the tower and into the ham shack is mil-spec RG-214/U, and that wasn’t changed.

Does the antenna actually “work” better, now? Really hard to say for sure, since I didn’t have both versions up at the same time. Performance seems the same based on observation. But the difference is the SWR is lower, and smoother, across all the bands. If I was using a solid-state amp or a fussy rig that cuts back TX power when SWR >1.5, this could certainly make a difference in the operation.

The original installation worked fine and having VSWR up to 2.0 didn’t really bother me; but it’s “better” now, and the only thing I changed was using about another two feet of coax to route it as shown in the photograph.

If you have or are thinking about an LPDA beam, you might give this method a try. It seems to work better, and it wasn’t my idea: I copied what I observed at commercial/government sites after visiting a local mountaintop.

-30-

Member Comments:
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Feeding an LPDA Beam Antenna  
by WA7KGX on June 23, 2014 Mail this to a friend!
The picture does not inspire confidence in the longevity of the floppy hanging coax.

Would it be better to tape the coax to the boom that is connected to the shield, and place the balun near the mast?
 
RE: Feeding an LPDA Beam Antenna  
by K6AER on June 23, 2014 Mail this to a friend!
The boom on a LP is balance feed for the whole antenna. The coax would have to be supported away from that boom.

Where Steve lives the worst weather they get is a smog alert. The cable will probably last for the next twenty years.
 
Feeding an LPDA Beam Antenna  
by NA4M on June 23, 2014 Mail this to a friend!
W8JI has info on the issue of taping coax to hot/split boom LPDA's:

http://www.w8ji.com/baluns_on_log_perodic_antennas.htm
 
Feeding an LPDA Beam Antenna  
by W6MQI on June 23, 2014 Mail this to a friend!
I fed my T-8 the way W8JI suggested didn't think the coax hanging down would last very long whipping in the wind. Also I did not install a Balun at the feed point mine is on the antenna mast per W8JI plan. SWR curves go from 1:2 to 1:5 20 through 10 meters the T-8 has far exceed my expectations.
 
Feeding an LPDA Beam Antenna  
by JOHNZ on June 23, 2014 Mail this to a friend!
Egads! 500 bucks spent on an antenna with little gain and questionalble VSWR, just to have the capability to operate on multiple bands? For the average ham, no way, but if you have money to burn, then I guess this can be your expensive toy.

For a ham, half that amount of money, combined with work and technique would be a far better investment, IMHO.
Granted, sans the multi-band capability of the LPDA.

Hams, at least old school, always put emphasis on antenna gain as a way to increase signal strength. Multi-band ops was a bonus only. Today's new amateur thinking, of course, is more, more, more. More power, more bands, more antenna, more bells and whistles, so I can trample on everyone else.

On the professional side, if you have interest in LPDA stuff, study up on Impulse Radiating Log-Periodic Dipole Array antennas using time-reversal technique, particularly as it involves feed lines.

 
Feeding an LPDA Beam Antenna  
by KA6MLE on June 23, 2014 Mail this to a friend!
I love my Tennadyne T6! I have read about using this technique to lower SWR... but it really hasn't been a problem for me. When operating digital in the 20 meter band and it starts getting crowded, it's nice to be able to drop down to 17 meters and get the same great DX in a less crowded band. I think my Log has great gain, especially compared to my parallel dipole!
 
RE: Feeding an LPDA Beam Antenna  
by W5WSS on June 23, 2014 Mail this to a friend!
I avoided the hot boom problems and instead opted for using Phenolic Flat stock plates for the element to 2"OD round Aluminum boom, mounting the elements to this Phenolic material served to properly isolate the elements in the LOG Periodic Cell along with a few feet of separation via a coaxial drip loop large enough to allow full rotation of the antenna and simultaneously also avoiding two problematic but different separate issues 1) Direct radiation or re radiation,(2) Common mode displacement currents problematic relative to the coaxial cable when it is routed on along the Hot boom. Both issues are better behaved by avoiding routing coaxial cable along the boom, especially If one does not want to add a second Choke at the junction where the line reaches the tower to route down.

Then a better performing technique is routing of the coaxial cable down away from the Apex feed point such as a large drip loop or referred to as a slack loop for rotating the antenna.

For the HOT booms as with the Tennadyne that uses an air wound choke coil and the coaxial cable is routed along the hot boom although looks to be better relative to longevity may be the wrong trade off. The drip loop sized well enough to allow full rotation lasts a long time perhaps any failure will coincide with a service maintenance schedule?

Now days a good practice For This Apex element feed method where we install a 1:1 current balun at the junction a 1:1 current balun to be enclosed in an conductive housing bonded to the shield driven boom portion is a pretty good solution when we want to leave the coaxial cable fastened to the Hot boom, otherwise the drip turn loop is desirable from a performance perspective and maximum longevity is slightly traded for a maintenance regimen instead.

The Aluminum enclosure of a DXE Bal-05-H05-a serves to bond the shield to the boom and places everything at the same potential up at the junction where the apex element is fed and is a good solution. A second device a common mode choke at the junction where the coaxial cable turns to head down the tower will allow the coaxial cable to be routed along the antenna boom otherwise a drip/rotate loop will work well to the same end.

Two devices up on the antenna and tower now days is expensive perhaps one can opt to build them saving some coin otherwise the alternative offers a decent contingency.

73
 
RE: Feeding an LPDA Beam Antenna  
by WB2WIK on June 23, 2014 Mail this to a friend!
I have a T6 and a T8, usually use the T8 in the summer months and put up the T6 in the winter when we have more winds. There's a difference in wind loading and performance isn't much different between them.

They work very well.

Any kind of horizontal antenna at 1WL above earth has substantial gain compared with "free space," so the first 6 dB or more is free. After that, the beam can add some gain. One big advantage to any kind of rotary beam is getting it high enough to actually work, and having enough F/B and F/S that you can notch interference and noise.

Anyone who's only used a dipole or a vertical and switches to "any" kind of beam notices a big difference...sometimes an astronomical difference. The difference between a 5 dBd gain beam and a 10 dBd gain one isn't anywhere near as obvious as taking that first step.
 
RE: Feeding an LPDA Beam Antenna  
by N4JTE on June 23, 2014 Mail this to a friend!
Your last statement Steve is well said and to the point!
Have not figured out the quote thing yet!
Regards
Bob
 
RE: Feeding an LPDA Beam Antenna  
by W5WSS on June 23, 2014 Mail this to a friend!
Yeah I really enjoyed my home made 6 element 10m Log/Yagi around 1983.

The same feeding methods for the pure Log Periodic apply since with the (My version) Log/Yagi there are 4 driven elements forming a Log Cell. in conjunction with a director and a reflector.

When I built mine, good 1:1 current baluns were of the low power variety mostly.

Core types were not fully refined as today.

Since as you know Steve The balanced split dipole elements are usually" designed with an optimal TAO and Sigma then the following longer elements are tuned and positioned to find a good compromise of all the attributes obtainable and my version was very effective relative to front to side ratio, front to back and forward gain along with the horizontal position relative to the earth surface and height.

Mine was a 50 ohm input impedance so that was a good match to RG-214 but needed a Balun, I chose to use a mono band air wound choke instead as back in those days the baluns were not ready for such rugged demands.

I used 6 turns and 10" diameter as I recollect and a large drip /rotate form. I admit I was concerned about the pivoting of the feed line as I turned the antenna but never had any intermittent opens or shorts or water intrusion into the line after 10 yers of service retired the antenna GRRR wish I could have it back up but no can do!

My Log/Yagi was home made simply because one could not just go out and buy one.

The Phenolic flat stock plates were a nice addition and I used 1/2" wide X 1/16" thick Aluminum strips to do the transposed element phase shifting.

The technique you have included should work to be a good revision.

73
 
RE: Feeding an LPDA Beam Antenna  
by K6AER on June 24, 2014 Mail this to a friend!
To mention what Steve had said about antenna height is very true.

I have a 4 element SteppIR at 105 feet. Along with other dipoles and verticals, I see as much as 20 dB signal increase when switching to the beam.

Hams make to much about antenna gain and fret wither antenna A has .2 dB more gain than antenna B. As Steve had mentioned, what is important is the Front to Back and Front to Side rejection.

When you signal is 30-40 over S9 nobody will see the 1 dB difference between various beam antennas. What your receiver will appreciate (as if they had feelings) is the rejection of unwanted signals roaring down the coax. The front to side and front to back will make a big difference in a DX pileup.

If you cannot hear them you cannot work them.

And,I might add, power is the next most important item after a good antenna. They cannot work you if they cannot hear you.



 
Feeding an LPDA Beam Antenna  
by VK3CHV on June 24, 2014 Mail this to a friend!
Website , W8JI.com , Follow the links , Antennas , Baluns on Log Periodic Antennas , Good Explanation .
 
RE: Feeding an LPDA Beam Antenna  
by WA7PRC on June 25, 2014 Mail this to a friend!
My new-to-me Force12 C-4XL (40/20/15/10m yagi, 30' boom) at 72'/22m seems to work quite well. While it's advertised as having 9 elements, it's functionally 2 elements on a given band. The driven elements on 20/15/10m use mutual coupling on one feedline, and there are two DE on 10m. 40m uses a separate feedline.

Elements for 20/15/10m are full size. Elements for 40m use linear loading. Bandwidth is nearly flat on each of 20/15/10m. Bandwidth on 40m is expectedly narrower -- about 100 KHz. For 20/15/10m, there are no adjustments. 40m has adjustable linear loading, and a lumped element Beta match. Following the instructions, it didn't take much effort to dial it in. Force12 says it can be used on 17 & 12m with a tuner. I noticed my TS850SAT works fine with it on 17m, and easily "dials it in" on 12m. Of course, I wouldn't expect the pattern on those bands to be good but, I don't operate those bands (no contests).

Front-Back and Front-Side seems quite good. Experiments with locals showed they were WAY down with the yagi pointed away from them, and nearly gone altogether with it turned 90° to them. I'm VERY happy with it.

vy 73,
Bryan WA7PRC
Photos on Flickr: http://www.tinyurl.com/wa7prc-tower
 
RE: Feeding an LPDA Beam Antenna  
by WB2WIK on June 25, 2014 Mail this to a friend!
The advantage of the LPDA design is bandwidth.

Not only does the typical one work 20-17-15-12-10m (and some do more than that) but they cover the frequencies in between, also...so for MARS work or whatever, they're not a bad choice.

But "tremendous gain" isn't one of their properties, until they're very, very long with a lot of elements.

The military and some industrial users have found them very useful for their frequency agility. I like them for how "dumb" they are. Pretty much nothing to fail, unless elements start falling off.
 
RE: Feeding an LPDA Beam Antenna  
by WA7PRC on June 25, 2014 Mail this to a friend!
WB2WIK: The advantage of the LPDA design is bandwidth ...

Of course. For those who can make use of the extra bandwidth, they're great. However, for me, the F12 C-4XL is exactly what I wanted. I especially wanted rotatable directivity on 40m. Stacking yagis to get 40m coverage or using other yagis that cover 40m and up would've been much more mechanically daunting and/or expensive. As it was, the used C-4XL became available at the right time and for a bargain price.

vy 73,
Bryan WA7PRC
 
RE: Feeding an LPDA Beam Antenna  
by W5WSS on June 26, 2014 Mail this to a friend!
REGARDING lOG periodic Arrays, I agree Steve Bandwidth. Forward gain holds to convention with the other attributes adjusted for a good compromise such as front to side and front to back ratios etc. and of course feed point ZO.

The TAO and Sigma is usually very close to 1 as a design parameter regardless so larger is normal when the design parameter chosen is optimal.

LB Cebik modeling work regarding LOG/Yagi antennas was exhaustive but perhaps not the final word.

He found that a Log periodic grouping of driven elements numbering from 2,3,4,5 forming a Log cell* that again can be optimized using the optimal Tao and Sigma design parameter yields more than simple band width but 5 being about the limit of maximum return relative to Gain as well.

The 5 element version returns excellent performance itself as about a 100 Zo structure.

The excellent cell can be operated independent as is* or further efforts to design some parasitic elements which build upon the cell structure performance and add more forward gain than can be found using the Log Periodic equivalent for the same space it occupies.

Simple engineered parasitic elements centered on the boom need only be dc connected since no voltage or current phase change occurs allows a neutral fastening to the boom.

I say all this because most do not think about the antenna as a really good performer where it finds it's natural home on 10m and with a reduction of about 50% per square foot of space yields rivaling performance to many larger longer booms Yagi's and even Quad Beam designed for that band.

My Log/Yagi for 10m was on a boom that was only 13.6ft long while all the elements were full size and it specd out as good if not better than the 6 element optimized yagi cousin.

Now the band width holds the performance of the Log Yagiand that advantage becomes really nice because the 6 element equivalent Yagi only performs equally on about half the bandwidth of the Log/Yagi.

I really like the hybrid Log Yagi on 10m.

73

73
 
Feeding an LPDA Beam Antenna  
by N5PG on June 29, 2014 Mail this to a friend!
Check out the Yahoo Group for these antennas. W5CPT has done work in this area. One other suggestion I've seen if you run the coax along the lower boom is to insert current choke where the coax goes vertical at the boom to mast plate. 73
 
Feeding an LPDA Beam Antenna  
by KB9UMT on July 5, 2014 Mail this to a friend!
Nice job as always Steve
 
RE: Feeding an LPDA Beam Antenna  
by JOHNZ on July 11, 2014 Mail this to a friend!
Ah yes, and now his adoring fans breathlessly await Steve's next missive.
 
RE: Feeding an LPDA Beam Antenna  
by K9IUQ on July 11, 2014 Mail this to a friend!
Johnz: WB2WIK has something you do not. The balls to use his callsign and not hide behind a phony name.

Having owned a T-6 for about 6 years I would have to agree with everything WB2WIK said. It is a decent low cost beam and not too big. My neighbors think it is just a big TV antenna and I do not tell them different.

I have over 300 countries worked on the T-6 - it seems to work.

Stan K9IUQ
 
RE: Feeding an LPDA Beam Antenna  
by JOHNZ on July 12, 2014 Mail this to a friend!
@K9IUQ

Gee guy, sorry your whole day was ruined by a neutral remark not even intended for you. Perhaps your aggression level was running a bit high, when you felt the urge to act on Steve's behalf?

I often do not agree with Steve but do find him to be an engaging person in face-to-face conversation, and he does quite well in defending his positions.

Steve and I are worlds apart on most issues, but I have always enjoyed debating him.

Be assured, Steve has a very thick skin.
 
RE: Feeding an LPDA Beam Antenna  
by WA2OLZ on July 31, 2014 Mail this to a friend!
My T8 is mounted about 12 feet above the roof, putting it somewhere around 40 feel above ground level. Far from perfect, but what is possible at my QTH. It works great! It replaced a Force 12 C-3SS multiband beam, a good antenna in it's own right. The T8 is far more effective than was the Force-12, mounted in the same spot.

I chose to use a pair of DX-Engineering baluns, one at the feed-point and one below the boom to mast interface. SWR is amazingly less than 1.3 almost everywhere except for a bump to 1.6 or so right in the middle of the 17M band. I have yet to try trimming it to flatten the SWR.

There is a lot of web controversy about the 'right' way to feed an LPDA. It never ceases to amaze me how some hams can get so agitated if you don't agree with their version of fact!

Bottom line - it works great for me, has endured Surper-Storm Sandy and a couple of the worst ice storms I've ever seen - and did so with zero damage. I wish I had pics of the elements bent down almost vertical under the ice! They returned to their normal configuration when the ice melted.
 
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