15 Meter Yagi: Gamma or T Match?

[w8khk]

I finally got a concrete base poured for the 35 foot crank-up AlumaTower, and I plan to put a three-element 15 meter Yagi on top.  I am trying to decide whether to use a simple gamma match, or go the additional effort of a T match.  Perhaps the experts here can help me decide.

A little background....  I used a three-element 15 meter Yagi made by Gotham for three years while in the Air Force.  I had a 30 foot guyed pole with rotator on the top of a flat roof, three story barracks at Davis Monthan AFB in Tucson.  The gamma match seemed to work well; if I could hear them, I could usually work them.  I worked lots of east coast hams, as well as most of Europe and Asia.  It had a simple gamma match, fed with almost 100 feet of RG-58.  I ran an SB-101, either barefoot or with a 2KW PEP linear, a pair of 4-400As running about 3500 volts on the plates. 

I will be scratch building the three element Yagi, using dimension tables in a recent ARRL handbook as a starting point for element length and spacing, centered on the phone section of the band.  Using gusset plates and clamps on the elements, everything will be adjustable.

I plan to run legal limit AM and SSB, feeding it with about 60 feet of RG-8, or better cable, depending upon recommendations.  Will the T match significantly outperform the gamma match?  Are notable differences expected both in receive and transmit?

If I build a T match, should I use coiled coax for the balun?  How much useful bandwidth should I expect with this feed, the entire phone section or just a segment of 15 meter phone?

An unrelated question:  What lubricant/antioxidant is recommended for the telescoping aluminum element sections?

Thanks in advance for your suggestions!

[w7fox]

I built a 3 el 15 meter yagi using parts from my Gotham quad that I never got working.  I used a gamma match and it worked really fine.  I use a hairpin on my 20 meter yagi, so I never had the pleasure of trying a T match.  Seems to me that everything works, even the gamma I tried on a dipole, so build whatever suits you.  I liked the gamma for its simplicity.  Enjoy hanging aluminum.

[KL7OF]

Gamma match will work fine...use insulating material that will handle some power and seal the ends of the cap section once you get it tuned...A vacuum cap works well for high power....Values for the cap are found in the handbook..
   I use the anti-ox for alum wire connections on all the joints..shine them up and put on a LIGHT COAT...available at home depot or similar stores or any electrical supply house.
Good Luck...Steve

[w8khk]

Thanks for the replies and suggestions.  Christopher, you are very close to where I used my Gotham Yagi.  Hopefully we can work each other once mine is up.  I just finished putting down 8 ground rods today, the tower will go up soon.

Steve, thanks for the suggestion on the Anti-Ox.  I found good details in the ARRL antenna handbook for the gamma match rod and capacitor, and I think I will use the sleeve and rod capacitor technique.  Although the handbook describes in detail the T and delta and gamma matches, it does not give any pros or cons of any of the various methods.  I have been wondering if feeding both halves of the driven element is more efficient than driving just one half, and parasitically exciting the other half.  I will probably just go with the gamma for now.

[W2VW]

An un-un wound from 7 or so turns 4" diameter of feedline cable wouldn't hurt anything right at the feedpoint.

Using one scaled down for 10 on my Yagi and the proof is in the null on local signals 90 degrees off main lobe. Knocking them down 28 dB on guess meter when wind is not blowing.

The idea is common mode current will wreck null.

Also used hairpin and will probably do so again if present antenna gets hurt. Splitting the driven element is a little more work when one is used to CB antenna hardware. Hey, it was available. All antenna designs here are temporary :  )

[w8khk]

Thanks for the suggestions, David.  

Although my Yagi experience is limited to the one Gotham 15m 3el, I am actually using the parts, ie, raw aluminum, from a Mosely TA-33 Junior.  Dad used that antenna, but the traps could not handle the power, so the traps are removed and I am making the elements from what remains.  

Initial dimensions I am taking from the 2003 ARRL handbook optimized Yagi tables developed by Dean Straw.  

THe TA-33 Junior used a hairpin loop, and of course the driven element was split and insulated in the center.  I thought of going this way, but that added too many variables to the starting point, so I will just go plumber's delight construction mode for the first pass, with the gamma match.  I will add the un un per your suggestion.  I have the test equipment needed to tune it to the center of the band.

I am hoping that, in addition to gain and FB, that I might also eliminate some of the vertically polarized noise that is so prevalent here.  

[W2VW]

Nice to be able to use legacy stuff from your Dad. He was a giant.

Having the hardware for that split driven element available would pretty much suggest a hairpin match.

All that gamma jazz is for easier mechanical construction with basic off the shelf parts as you know.

Both take time to set up and both perform equally when all is correct.

50 ohm "ugly" BalUn on one band is foolproof. Hairpin stub can be made from a wide variety of material. As an example spare 10mm hardline can be used at 10 meters for the hairpin. Symmetry is the word. Plan on some outside time with suntan lotion. 

Hope to run into you on 15 when mother nature allows.

[w7fox]

I forgot to mention that I took the hairpin off the 20 mtr yagi and feed the driven element with home brew open wire line.  It works great on 20 and makes a fine rotary dipole for everything above.  I use a short piece of 200 ohm twin lead at the rotor.

[w8khk]

David, thanks for the compliments regarding my dad.  And the suggestions on Yagi techniques.  Christopher, I thought of using open wire on the Yagi, as I have done with my 132 foot dipole with an ugly tuner.  But for the antennas on the tower, I want to use a coax relay to select either the 15M Yagi, the 2M vertical at the top of the tower, or a 40M dipole with balun at the bottom of the mast, under the Yagi.   To avoid interaction, I plan to use a non-conductive fibreglas boom for the Yagi, and place the 40M dipole in parallel with the Yagi boom.  I will just have to remember that the rotor control pointer is 90 degrees off for the dipole, which of course will be bi-directional. 

Tower grounding is completed, next I need to mount the rotator with its control cable, and make sure it is aligned so the mast is free to rotate in the thrust bearing, then the tower can be raised.  After a trial run, I will lower it then mount the antennas.  Should be fun to operate compared to the old dipole!

[Steve - K4HX]

Another possibility.

http://www.qsl.net/dk7zb/start1.htm

[K1JJ]

Hi Rick,

Most of the various Yagi matches will give the same result in the end, assuming that there is no imbalance generating shield currents.   Most of the matches have stood the test of time.

For my homebrew Yagis, I prefer the balanced "T-match".    I use a split, insulated driven element and an adjustable slider on each side.  The T is fed with a balanced 4:1 step down coaxial balun that accepts the 50-75 ohm coaxial feedline.    Usually, my Yagi is designed for high gain and low impedance (15-20 ohms) which allows  the 4:1 step down and adjustable T to give a nice 1:1 75 ohm hardline feedline match.    Details for a  balanced 4:1  1/2 wave coaxial balun can be found on this website.  (No ferrite used)    The driven element swr  R+J  is adjusted using BOTH the T-match  sliders and the element end tip lengths.  


The exception is my 2el 40M Yagis  using a 50 ohm input impedance needing only a direct 50 ohm coaxial feed using ferrite sleeves and coaxial turns on a PVC form for isolation.  The driven element swr is adjusted using the element end tips only.

In general, the lower the input impedance and the higher the gain and bigger the transformation - the higher the Q and narrower the bandwidth. Thus, my 40M 50 ohm Yagis are relatively broad-banded, whereas the low impedance Yagis using the T match and 4:1 baluns are narrower. But on 20-10M, this is not a problem.   The wider bandwidth is needed on 75 and 40M percentage-wise.

T

[w8khk]

Tom, thanks for the details regarding the various feed options and their relative merits.  I think I will go ahead and use the gamma match initially, but I built the antenna so the driven element can be split, not grounded, that way I have the flexibility to play with different options. 

Today I finished grounding the tower base, and made gusset plates so the elements are mounted on the boom, and the boom to mast bracket is also completed. 

I will start with just the 15M antenna, and later add the 40M rotary dipole, and see if it has any negative impact.  I do not want to raise everything too high right now.  I also have a yardarm on the top of the tower below the mast, with a pulley and lanyard so that I can pull up a wire antenna using OWL.  The trees have not been too cooperative with the frequency of storms this spring and summer.

After I get the feed installed and tuned, will do some on the air testing.  Thanks all who supported me in the planning of this project.

[w8khk]

Progress report and another question... or two!......

The tower has been raised, and the upper supports to the chimney are extremely solid.  The ground rods have been connected to the base-plate under the tower base using #4 AWG copper, with lugs crimped and soldered, then bolted to a 1/4" thick aluminum base-plate, about 14" by 16".   This will be tied to the house water pipe ground at the service entrance before any coax is brought into the house.  There will be an angle plate mounted to the base-plate, and dual-female threaded N connectors will be mounted to the angle, so that each antenna coax line will be grounded at the base of the tower prior to entering the building.  

All the yagi elements have been assembled to the proper initial length, and the gussets with u-bolts and muffler clamps are assembled as well.  The boom will consist of two 1-1/4 8-foot fibreglas tubes at each end, with a single 1-1/2" fibreglas tube in the center.  The mast is a 1-1/2" fibreglas tube, with a 1-1/4" fibreglas solid rod in the center.  All fibreglas components were purchased from Allen Bond, Max Gain Systems, WB4GNT.   He is a fabulous fellow to deal with, provides lots of good advice, has very fair prices, and delivers promptly.

I am now ready to test the yagi and adjust the gamma match.  Initially, I will use a standard variable capacitor and aluminum tube to reach a 50+j0 match.  Once the impedance is determined via the length of the gamma rod, I will remove the variable capacitor, then use an additional 5/8 inch tube insulated from the 1/2 inch gamma rod, to duplicate the required capacity.  After adjustment, I will seal the ends and provide a type N connector to attach the coax to the driven element aluminum gusset plate.  

So here is the question:  Since the feed is unbalanced, fed with 50 ohm RG-8 with the shield grounded to the center of the driven element, is there any need for the coiled coax balun, or should the feed run directly down the mast with no balun?  I understand a T match requires the balun, but I do not understand whether there is any advantage to the balun with the classic gamma match.

Directions I have read for adjusting the match recommend orienting the boom vertically, with the reflector on the ground, pointing the director element skyward.  Then adjust for 50+j0 match.  I know that F-B or gain measurements cannot be made this way, and adjusting the reflector or director later will impact the driven element match.  I am not looking for an exceptional contest antenna, just one that will allow me to enjoy 15 meters with minimal noise from a horizontally polarized antenna, with reasonable directional capabilities.  Does anyone have any better suggestions for initial adjustments?  I wish to raise the antenna on the tower a minimal number of times, so having the best method and pointers is very valuable to me at this point.  I can do front to back and gain measurements when the antenna is raised to normal height, but it will be difficult to adjust element spacing or length at that point.  Any suggestions?

 I am using the measurements from the 2003 ARRL handbook, page 20-31, model 315-12, which refers to a 3 element 15 meter beam, with a boom length of 12 feet.  I have 16 feet of boom length available, and I would like to optimize the element spacing.  With a 12 foot boom, the table suggests 48 inches between the reflector and driven element, and 92 inches between the driven element and director.   Initial calculated element lengths are Reflector: 290-3/4",.  Driven element: 267-1/4", Director: 255".  Any pointers or references for optimal element spacing?

Since I have a non-conductive boom for the 15 meter yagi, I believe I can mount a rotating dipole about  six feet below the yagi, in parallel with the boom, and avoid any undesired interaction between the two antennas.   The dipole will consist of two separate vertical marine antennas, mounted end-to-end horizontally just above the rotor thrust bearing at the top of the tower.  The yagi will be at the top of the fibreglas mast, just below the 2 meter / 70 centimeter vertical at the top of the mast.  The marine antennas are fibreglas units about 12 feet long, 2 inches in diameter, with a 9 foot fibreglas extension similar to a ten or eleven meter mobile whip.  This will provide a 34 foot long dipole, but the element conductors are spiral wound, so it may work on both 40 and 75 meters.  I have not yet decided whether to feed that with a bead current balun and RG-8, or use a section of window line then open wire from where it attaches just below the rotator section of the mast.  This antenna will be parallel to the non-conductive boom of the 15 meter yagi, so they should coexist peacefully.  

After I get the gamma working properly, the antenna should be raised in short order, then all the lines need to enter the house via a junction box with disconnects, fusible links, and relays.  I will start testing with sideband, using the TS-830, then when everything looks ok, the Viking II will be loaded up, using the 75A3 or 51J4 for receiving.  

The attached photo shows the tower mounted and raised, but not elevated.  Rotor and cable are installed and operational.  I will lower the tower one more time to install the yagi, rotary dipole, and the vhf/uhf antenna at the top.

Thanks in advance for any comments and suggestions.......

Note:  edited to correct errors in yagi element length dimensions.

[KD6VXI]

The gamma match,  unless placed correctly,  will skew your beam pattern.  It's an unbalanced ed feed.

The T,  by  nature of it being balanced feed,  doesn't.   Neither does the direct split driven design.

Your dad helped me with mine.   Hope the info helps you :-)

It's not that it makes the beam unusable,  just your lobes aren't textbook.   You may never notice......

[Steve - K4HX]

Coax is unbalanced. 

[w8khk]

Coax is unbalanced. 

Exactly!  That is why I see no need for any beads or coiled coax with a gamma match, where the coax outer conductor is at ground potential with respect to the driven element. 

So if the information I have read regarding adjusting the gamma impedance and reactance to match the cable is correct, then it should work well when mounted on the tower.

[W2VW]

The gamma match  uses an unbalanced source to an unbalanced load and no balun should be required.

Any RF that happens to find its way onto the outside of the feedline shield will result in pattern degradation. Normally this isn't a problem.

I use an ugly balun on my gamma match in case. Its so easy to construct for 10 meters and I'm used to making them for hairpin matches.

Surface wave horizontally polarized signals can be DFed with stunning accuracy. The 90 degree null is 28 dB on the guess unit scale. Helps with some of the people who drift onto the band. 15 meters shouldn't have this kind of activity though.

[w8khk]

The gamma match  uses an unbalanced source to an unbalanced load and no balun should be required.

Any RF that happens to find its way onto the outside of the feedline shield will result in pattern degradation. Normally this isn't a problem.

I use an ugly balun on my gamma match in case. Its so easy to construct for 10 meters and I'm used to making them for hairpin matches.

Since there is no disadvantage to adding the ugly balun, I will install it just to be on the safe side.  The antenna is almost finished and will go up soon.   Thanks for the help!

[w8khk]

The initial gamma match tests look promising.  The gamma capacitor consists of a tube within a tube, with polystyrene insulation.  Excess tubing will be pruned after final adjustments. (The half cliplead is not part of the final design.)

[K1JJ]

Hi Rick,

I have two suggestions based on mechanical considerations...

It's difficult to see from your picture, but once the match point is determined, be sure to put a screw and nut thru the gamma slider and tubing.  They can easily work loose in the wind otherwise.

Also, I've found the weak point of most installations is using coaxial connectors out on the antenna that are difficult to service without taking down the antenna. Splices, especially connectors in the weather,  suck.  Moisture/ condensation gets in and/or a high swr flashover occurs due to ice on the elements, etc.   Instead, I always make a coaxial pigtail, seal it well, add silver plated  lugs and directly bolt  the coax leads to the elements. No connectors.  Don't worry about swr since you will be matching it anyway.  

Seal the coaxial connections real well using clay, tape, wx sealent or whatever - and the splices will never crap out, even with high power.  I have some of these connections still up after 12 years and more. In contrast, the coaxial connectors seem to last about 5-7 years and then need to be pulled apart and cleaned or replaced, no matter how well I seal them.

I realize most commercial Yagis use SO-239 connectors on the driven element, but the direct approach will last longer, considering replacing usually means taking the antenna down and we are all getting older.... :-)



T

[w8khk]

Hello Tom, 

Thanks for the suggestions.  I was planning on mounting an "N" connector on the gusset plate, and using a #12 wire to connect it to the gamma rod.  I never use PL-259 or SO-239 connectors, but I figured the N connector would be waterproof.  But I will take your recommendation and use silver plated lugs and coax-seal before I raise the beam.  I will use the N connector for testing until I get close to raising the antenna, then the coax with pigtails.

I read your references on this forum regarding the ugly balun, and will use 5 turns of coax with a diameter of 4-1/2 inches, solenoid wound.  I believe I can wind five turns, using wire ties, without using a piece of PVC pipe as a form.  But if you recommend the form instead, I will go that way.

My tower is an AlumaTower that dad had at his shack, two 20 foot sections, when raised it is 35 feet to the top of the thrust bearing, with five feet overlap between the inner and outer tower sections.  It is rather easy to raise and lower, but I of course want to do that a minimum number of times.

 One of my HP colleagues has recently become a ham, I mentored him and his son, and helped him fully restore a National HRO 50T he purchased at the Ellijay GA fest this year.  He has been working with me on pouring the tower base, installing ground rods, rotator, and test raising the tower. 

Today we completed assembly of the yagi, which originally was a TA-33 Junior from Mosely.  Traps are gone, it is now a full-size three-element 15 meter beam.  My only concern now is whether the polystyrene insulator in the gamma capacitor (concentric aluminum tubes) will withstand legal limit.  I am not sure whether I should hipot that cap before raising the antenna.  I am also not sure what voltages to expect.  Right now I plan to use only the phone portion of 15 meters, but he is getting me interested in digital as well, so I may be using the entire band.

I will have a run of about 90 feet of RG-8 for the Yagi, and about 100 feet of RG-8X for a 2M/70CM vertical just above the Yagi.

There will be a 36 foot rotary dipole mounted under the Yagi, fed with open wire and spreaders, mounted just above the thrust bearing, just to experiment on the lower bands.  I am blessed with a very high noise level in the neighborhood, and if the dipole does not help with reception, I will try some loops or deltas.

By the way, I am still awaiting parts from Steve for the Class E rig, and will make that a summertime transmitter.  I will start of with his PDM generator, and tube modulator/final, and later make a 100% solid state class E rig.  In the meantime, I will get my grandfather's PP 450TH rig with 810 modulators on 75, that should work much better than the PW Viking II.  The tower will hold up the dipole/inverted Vee much better than the trees.  Storms have really created havoc here!

73, Rick

[K1JJ]

Hi Rick -

Sounds good.

Yes, the coax pig tail will work out well for you.   Unless a connector is sealed in a vacuum, the existing air will have some moisture in it. If it's a humid day, then this moisture gets sealed in and is waiting for a cold day to condense inside.  Over time, moist connectors tend to form a green yucky paste on the pin / socket connection and go intermittent, especially on receive. They eventually flame out under high power. A direct connection eliminates this problem. At least this is my own experience after building and using many Yagis over 30 years.


For the coiled coax balun, wind five coax turns on a 4" - 6" PVC tube and you will be OK on 10-20M.  We shoot for X10 the Yagi input impedance, so five turns should be at least 500 ohms inductive reactance on 14 Mhz.... and higher on 10 /15 M, which is a good thing.

Also, for the least inter-turn capacitance, use a 1/4" rope spacer that gets wound on with the coax. Easy to do and produces an excellent coaxial balun.

T

[W2VW]

If using no connector the gamma capacitor can be fashioned from some aluminum stock that fits snugly on the coax center conductor. Capacitance will be close to spec for same length of cable.

So many ways to skin a cat.

[w8khk]

For the coiled coax balun, wind five coax turns on a 4" - 6" PVC tube and you will be OK on 10-20M.  We shoot for X10 the Yagi input impedance, so five turns should be at least 500 ohms inductive reactance on 14 Mhz.... and higher on 10 /15 M, which is a good thing.

I am going to try winding the five turns space wound without the pvc form, using wire ties to retain the proper form.  If this does not work, then I will add the pvc. 

Pigtails will be used, no connector.  I am hoping the gamma capacitor section can handle the power.  Will not know until I crank up the 2KW linear.  Most important is to be able to hear the stations I work, and having a horizontally polarized directional antenna should make a big difference here.

Going to Florida for a few days, so will get back to finishing the Yagi when I return.

[w8khk]

If using no connector the gamma capacitor can be fashioned from some aluminum stock that fits snugly on the coax center conductor. Capacitance will be close to spec for same length of cable.

So many ways to skin a cat.

thanks for the suggestions.  I have seen sections of coax used as the capacitor for the gamma match.  I am initially going to try the concentric sections of aluminum tubing with polystryene insulation....  If this does not work, I will probably go with tubing with more space between the inner and outer tube, then use teflon insulation.

I initially tested the gamma with a variable capacitor, then measured the cap with a bridge.  I adjusted the tubing gamma capacitor with the bridge, then installed it on the driven element.  Initial measurement was an almost perfect match!  The only question is whether the gamma capacitor can handle QRO voltage rating.  We shall see.  The tower, fully raised, is only 35 feet, so it will be no problem to replace the gamma cap if it misbehaves.  Thanks all for the suggestions!