Perspectives on GAP Antennas
The contents of this web pag is an accumulation of comments received from QRP-L members concerning their experiences with GAP antennas. I too have a GAP antenna (Titan) and enjoy it very much. I found this dialog interesting and useful to me in understanding my antenna's operation and performance ... and thought others could benefit too.
73, George N2APB
PS: The information presented here is not guaranteed to be correct, nor is it endorsed by GAP Antenna Products. Antenna discussions are usually embellished by folklore and biased by personal experiences. Use this information at your own risk! ;-)
The Gap vertical antennas are all simply dipoles, but vertical
dipoles using low loss linear decouplers for loading the different bands. These decouplers
are akin to stubs of open wire line we have used for years as matching devices on various
types of ham antennas. They just look more exotic when one side of them is the vertical
mast itself. They effectively form capacitors to shorten various parts of the tubing to
resonate on the higher bands.
The use of the asymmetric elevated vertical dipole is a common Broadcast Band antenna in places such as Argentina, and a paper on such appeared in the USA IEEE Antenna Transactions publication some years back. I got a copy of this paper from the Gap booth at Ham Com one year . For Broadcast work, you want to minimize ground losses, and have a good pattern in your coverage area, which the vertical dipole brings without investment in radial systems and ground screens. When you put the (40M) counterpoise wheel on the bottom of the Gap Titan, you end up with not needing the same length each side of the center insulator. Inside the Titan, (and I think other Gap models), there is a coax stub for loading on the lowest band. To make the coax stub fit the space inside the tubing, you have a capacitor across its upper end, with one side and one side of the coax tied also to the upper dipole end. This matches the antenna to your feed for 80M use over a greater than 100 kHz band. Other bands are full coverage. (less than 2:1 SWR)
Does it work? Yes indeed. As long as it is not coupling to something in the near field it seems to bring a lot of nice QSO's and DX with its low angle characteristic. It is stuck on a TV mast 6 feet above my back yard. As a bonus, I have enjoyed good short skip QSO's within the state, or in nearby states on 20M, where short skip was rare in the days I used low dipoles on 20M. I have heard of interaction as with any vertical, if you have something like a metal flue chimney nearby of a resonant length like 30M quarter wave. Its bandwidth on 80M exceeds the specs, thus I have not used a tuner for any band. It is full band coverage on 40M and up, and a tuner is not even recommended. It is quite study, having survived 45 mph winds that I was able to document, and more recently, some straight line winds that felled taller trees and limbs all around it during the Jarell TX tornados up the road from me. The bottom tubing is triple walled, and thus requires a couple of big folks to walk up, or three ordinary folks like most of us.
I had an "all band" Hy Gain coil trap vertical before with ineffective ground system. (I have mostly rocks under thin soil). The Gap works much better, although a home made vertical could be made, for less money IF you had a source for the aluminum tubing. (A BIG IF these days!!) If your time counts for something, Gap solves the all band problem with a minimal investment of time. However, I would like someone who has the time to sometime create a home made vertical dipole set for all these bands, and see if the performance is as good or ? I suspect the use of large tubing really is the full band coverage secret, and just wire verticals hung from tree limbs might not equal that aspect. But, one could use a cage of wire, and solve that issue.
Hope this inspires some antenna experimenters. I did not hear about any Gap beams at Ham Com this year and I always ask if I can be a beta-test site when I see Richard! I think the Sommer beam does use this type of loading/decoupling for band changing. That is a mighty beam, and some day, if > I ever get the space----
72 Stuart K5KVH
The Gap Titan is intended to cover all bands above 40 M with 2:1 swr at ends of the coverage, and very low swr in each band. No adjustment is needed for most installations, save the 30 ft. wire counterpoise.
there is an adjustment for the counterpoise wire length to move your best swr point on 40M around. Of course, as stated in the instructions, a capacitor on the end of the coax stub inside the mast sets the 100 kHz segment of coverage for 80 or 75 M, and you have to have the cap for the section of band of interest.
The outside the vertical dipole mast, linear decouplers are lengths of open line, forming stubs on the outside of the mast, using the mast as one side, and the rods as the other for the higher than 40M bands. One of these has a screw and telescoping section allowing fine tuning. I think someone on the list some years back mentioned what band this adjustment was for, and it was either 30M or 15M I think. Check Gap Titan in the archives of QRP-L. Check the total length of the rod that adjustable piece is on, and see on which band it is closest to quarter wave. (I think it is possible that some rods affect two bands, ie 12 and 10M are probably one rod.)
One could fashion a new piece of aluminum tubing to slip onto any rod end, to lengthen it thus lowering or raising frequency, while observing the effect on SWR. Try it on each rod in turn, until you get an effect on 15M where you want lower resonance. That should define the rod you need to adjust. The effect you get on the SWR vs. frequency, might indicate that a lengthened rod raises the resonance, but that is OK, you will have some effect and be able to determine which way to go for your desired result.
Another point is that I believe the antenna was said to be 3 meters off ground, close to a resonant quarter wave on 15M, and thus, try changing the antenna height by using more base pole, or less to possibly affect 15M results. Mine is about 2M off earth, and works over all of 15M very satisfactorily, but then I do not mind 2:1 swr, as its effect on DX is negligible!
Finally, although I never checked this, as 40M is 1/3 of 15 M, perhaps the changing of the 40M counterpoise interacts with the 15M resonance as well? It might be worth an experiment to see if some compromise setting of the 40M wire will help your 15m resonant point.
Most of the Gap Titans I have seen or heard on the air work very well indeed without any adjustment except the 40M wire and the choosing of an 80M capacitor at time of set up. I hope the ideas I have listed would help anyone with this type of linear decoupled antenna. Remember it is an asymmetric vertical dipole. That is, one end is shorter than the other to take into account the earth effect on that end closest to earth, but other wise it is just a dipole whose band of resonance is changed by stubs parallel to it.
Hope this helps,
Stuart Rohre K5KVH
Just had to add to the satisfied Gap antenna owners, and specifically the Titan model, the first all new commercial vertical I bought outright. (Others have been used, or club owned, etc.)
Really surprised at the comment from the 6 call who had a "flimsy Gap" and bad experience. Could not have been a Titan model!
However, for the education of those wondering about the models, I have worked with a friend and put up one of the earlier models for him some 7 years? ago, which had a V (five) in its model name. It was about the same weight as a Hy Gain 18 AVT I had up for a few years, which was replaced by my Gap Titan. Also, the earlier model I would guesstimate was about the weight and construction of R5's and R7's, (Cushcraft), which I helped friends and Field Day crews put up. (R5 &7 and the MFJ's don't cover all of the bands like 40M and 20M where the Titan does.)
Now, when I got a Titan DX model, I got the surprise that it was the heaviest vertical I had ever handled! That was because the lower half is a multiple wall aluminum tube of Aircraft grade.
It is an Asymetrical Vertical Dipole, as the article in an IEEE Engineering magazine calls this class of dipoles. As a dipole, it has two sections, just like your horizontal dipole, or a full half wave sloper dipole. Now for a half wave sloper you don't use radials, right? Nor for a horizontal dipole? Now why would you for any other half wave antenna, that is fed at or near the middle? Once you understand the premise of the Gap Titan as a half wave antenna, but just turned 90 degrees to the way we are used to the majority of dipoles, then you can understand the other design concepts it so cleverly incorporates. First and formost, at 25 feet it is a tad short to be a half wave on 40 m, or even 20 M. Enter the "upside down capacity hat", or 'counterpoise wheel' as I have also called it. This is the radial like wheel structure the Titan has on its lower end, which consists of a full quarter wave of wire for 40M as the circumference conductor insulated on some spokes sticking out from the lower end. Now one end of this wire is electrically connected to a spoke which is electrically clamped by a mounting plate to the lower end of half of the dipole. That makes the lower part of the antenna below the insulating gap section something like 45 foot conductor, part vertical, and part coiled as one big turn. The end of the coil comes back and joins its start through an insulated section. It makes a substantial radiating device when compared to say the Butternut antennas, (which I have also installed and used), and which are quarter wave verticals that do need a counterpoise or radials to 'complete" the other half (of the dipole) they are not.
Now, back to the Titan. It has as much or more bandwidth on 80m as the Butternut, and much more than the Cushcraft models. True, the total vertical sections are 25 feet; but that is a tall Bug Catcher, and the capacity hat or counterpoise wheel must help as well. No Bugcatcher has this 100 kHz bandwidth without a tuner, conservatively. (Now all you mobile hams don't ge tme wrong, the Bugcatcher is a FINE mobile antenna!)
To make up the 50 ohm match at the insulator between the dipole sections on80M, Gap uses the old tried and true stub technique of a coax quarter wave piece across the feed point. Cleverly, it is inside the upper tubing, which probably helped the tubing survive the tornadic winds my Titan went through during the nearby Jarrell tornado in May. To shorten the amount of stub needed, they place potted ceramic caps in parallel with the upper end of the stub. One side of that end of the stub also connects to the upper end of the top dipole half tubing.
Now you hopefully have the full picture. A thickwalled lower section, joined by an insulator to a flexible, but damped for wind vibration, upper section. It is limber enough to give a bit in a wind, and swing back to vertical. However, with all that weight at the lower half that joins to your pipe mast or ground bracket, if you get a wind gust, you have a big heavy lever arm which might loosen your pipe you provide to anchor in the ground. My TV mast was lighter weight than the Gap tubing, so I beefed it up on the side away from prevailing high winds here with a steel angle iron driven in the ground with the TV pole. You could use a field fence post for the same effect. Just u-bolt these to the mast you are using to hold up the Titan. My mast is only six feet tall, but before I guyed the Titan, a 40MPH gust pivoted the antenna into the branches of a nearby oak tree. My undersized base tubing just laid over, but the Gap straightened right up when I pushed back my TV mast, and hammered the angle iron into the ground. Nothing on the linear decouplers, (that look like trombone slides), was damaged. (These are the tuning elements that replace lossy traps and inductors with open tranmission line sections to resonate the various tubing sections to the various bands, another innovation of Gap.)
Since that experience, I added three guys of Wal Mart nylon rated at 880 pounds each. I just tied them to the tubing near the center insulator with a clove hitch. Nylon does stretch, thus, when it does, I go out and take up the slack with old Boy Scout knots. Even if I don't take up all slack, the guys keep things from whipping around, as they did even slack, during the May storm. Next door, a couple trees were detopped. My Titan is in the back yard, but in my front yard, the winds took 3 large limbs off a 21 year old pecan tree. Down the street, one tree was laid over at the base. On estimate was we had 70 mph winds during the tornadic storm. I think Skywarn calls these straight line winds, and it looked like a big club had come down the street swatting the trees. But the Titan rode it out just fine. I have never had to retighten any of the clamps that mount its decoupler insulators, nor any of the main tubing joints.
The antenna has been an amazing performer working both the expected DX, and more frequently than not, close in skip as well, such as New Mexico and the Texas Valley on 20M. On 80M, it has worked into Kansas, around Texas, as well as longer skip. It has outperformed all my other vertical experiences, while at a low point of the sunspot cycle.
Now, as certainly the price is a factor for some, I would hasten to point out that you could build your own tubing, (or even wire) vertical dipole, (if you have a high tree limb to suspend the wire). You could parallel some dipoles beside it for the other bands, as in the "sleeve fed" beams where one dipole is fed, and others parasitically excited. It think you could load the dipole on one end, with the wheel idea, and stub match the central feed, to get on 40 and 80M. I am kind of sorry I did not know about the asymmetric vertical dipole as a class of common AM Broadcast antennas in South America, before I bought the kit, for I think with some effort, one could do a wire version for certainly a lot less money.
Finally what swayed me was the materials availability of aluminum tubing was not good in my area, and the time factor. Time is money, even ham time. I paid good money, but I got good value, and George's clever ideas implemented with me doing some very minimal labor.
The Gap folks, Richard and George, that you see at the shows, are helpful and quick to fix things like the loss of some hardware out of my box due to overeager summer UPS help during shipping.
Another local ham friend bought the Voyager, and has great results with it on the lower bands it covers. But at 45 feet tall, it was more than I could handle in my small yard. It certainly is to be considered if you are mainly a 160 and 80M type.
Hey, the whole company signed and sent out Christmas cards this year! Now that is a first! Whatever you do, enjoy your antenna!
Stuart K5KVH (not a stockholder, but sometimes wish I were!)
As a Titan owner who really dug into its design background, and visited with the author of its design, I can tell you that each and every part is essential to its overall proper operation.
To remove the "counterpoise" would be to remove all resonance on 40 Meters and would undoubtedly detune other bands. Normally only adjusting the length of the 33 foot wire on the circumference of the counterpoise is all you do to "tune" a Titan. There was one linear decoupler with an adjustable segment, but my instruction sheet listed it as factory set.
The Titan is a vertical dipole that is made into an 80M to 10 M antenna by a couple of clever basics of antenna practice. One is to get resonance on 80M from a 25 foot dipole, they connect a cap loaded stub of coax across the center dipole feed point. The coax stub is about an electrical wave fraction with its cap, to match 80M at 50 ohms. Next, to get 40M form this short dipole they add an end element which we called a counterpoise, but is really an extension of 33 feet coiled into a circle. That's right, if you trace its circuit out, it joins the lower end of the dipole, and is electrically isolated from the radial rods that support it until you get back to the rod that electrically joins it to the lower end of the dipole. It is insulated from shorting to itself by the nylon line that you adjust to rubber your 40M SWR dip. The dipole sits on a double insulated plate to keep it isolated from your bottom mast. (You provide the support mast for elevated mounting.)
The linear decoupler rods are stubs of transmission line in action, and decouple parts of the 25 foot dipole for resonating in each band, 30, 20, etc. up to 10M. Not a lossy trap in sight, and a neat antenna! Mine only shows a single upper mast end above the roof line, as it sits 6 feet off the back yard surface. Got my card from Easter Island operation in 95 today, thanks to a slow buro (burro?) envelope, and that was done with the Gap in the pile ups at close of that DXpedition.
The poster has been advised by me to put up a mast high enough to clear the slope of the roof, with the Titan wire portion. I like to call it an upside down capacity hat, but calling it a counterpoise is easier for most hams to understand its action.
I "suspect" you could deform the rods to have the plane of the wheel slanted rather than parallel to the earth level, and it might still work on all bands without tweaking, but I would have to ask the Gap guys at Ham Com. You could thus, make the wheel clear the slope of a gable roof, by almost paralleling the slope.
GL, Stuart K5KVH
You asked about the Gap antenna line. Most of their models are verticals of a type used in South America for AM broadcast antennas, and known as an "asymmetric vertical dipole". This means, the two dipole halves are not equally sized. One side must vary because of it being closer to earth. With the addition of linear decouplers, (stubs), for the bands desired; you have a vertical dipole on multiple bands.
Gap does a neat trick with the 25 foot long Titan, that I have. They enclose in the tubing a capacitor loaded coax stub, to make it resonate on 80M as well as all other bands 40M and WARC up to 10M. Perhaps the most clever thing is how resonance is obtained on 40M. There is a set of radial rods at the bottom that support a circle of wire. The 33 foot wire makes electrical contact with the end of one rod only, that joins it to the main antenna. The wire circle is not completly closed, but has a nylon string making the last few feet of the circle. Thus, you have a full size quarter wave 40M element in a much more compact area, and it seems to radiate just fine.
When I assembled my Gap, I used an antenna analyzer to see where each coupler was contributing a band resonance, and was mystified as to how 40M worked, as none of them resonated there! When I got the antenna up vertical and went to put on what looks like a kind of counterpoise wire circle, I found my 40M resonator in that wire.
Mine is up 6 feet, and works more DX than my dipoles, G5RV, etc. It is amazingly good on occasion into the next state, but it is low enough take off at six feet to be a good DX antenna in this location. I am sure with poor ground we have, (rocky) its dipole being independent of needing earth return radials is a plus. It is a two piece, balanced antenna, although it is an electrical balance, rather than equal dipole halves. What really is helping it out is the Fresnel region out some wavelengths from the antenna. There is a clear shot to Europe, and to Asia, and over the North Pole. However, there are hills to the South, but I still work all the way down South America very well to Chile/Argentina, and into the Carribean to the Southeast. There was no tune up required like the MFJ vertical, and only one measurement to set my desired 40M resonance.
No Gap I know of is a cloud warmer! The quarter wave verticals without radials may be cloud warmers when you do not have good RF earth under them, or sea water.
Gap has out an interesting new antenna, for the patio confined antenna farm. It is like the Force 12 compact dipole. They "squash" the dipole down and make a cage of the ends to load the thing. I have not worked any yet, but would be interested to hear how anyone has used one.
It is easy enough to make your own parallel vertical dipoles of wire, and one central support. You might try it. After I spent my money, I realized there are some opportunities like that going begging. I made a 27 foot PVC mast for 99 Field Day, that has possibilities for verticals, if you guy it well. It is pretty spindly as you raise it. (I was using it with my horizontal Vee beam leg and two back stay guys.)
Another antenna for all bands not to be overlooked is the inverted extended Double Zepp (IDEZ) I did for field day. With ladder line feed and a mast, it can be tuned on all bands. Of course, you have to have a big lot for it, for its legs are each 159 feet. That is one with lowest band of 80M, and if you make it only 159 feet overall, fed in middle, it is good 40M and up. Gives gain, and is cheap. Insulated ladder line is reasonable from all the antenna cable companies, and larger mail order ham stores. It seems to work as well as the true open wire, (uninsulated) ladder line that seems discontinued. Saxon did make it but its insulators did not stay on well.
I tried making my own of #24 wire and some plastic clips sold with two nails for holding down house romex wire. Take the nails out, and you have a ladderline insulator, but if your time is worth money, like most folks, you will soon tire of threading these little spaces onto the wires. Been there, made some, it worked, but was spindly and hard to handle. Give me the plastic covered type, and slightly more loss! I could never see any measurable loss with window line- the type most available in North America. (Check out the Wireman's web page.)
Antennas and QRP kits are the last refuge of the technically inclined hams, glad we have a lot of each to play with!
72, Stuart K5KVH
GAP TITAN TUNING
Hi gang, I asked GAP the following, "Which bands do the adjustable tuning rods cover?"
Their answer was:
The slotted extender by itself controls 12m.
The slotted extension with the 23" extension below it controls 15m.
And, the capacitor (CAP) in is as follows:
Info from BUD, KV7G...
Color Value Freq.
Black 3300pf 3.5MHz
White 2800pf 3.6MHz
Red 2300pf 3.7MHz
Blue 1900pf 3.8MHz
Green 1500pf 3.9MHz
Clear/Orange 1400pf 4.0MHz
I suspect they are silver mica capacitors.
72, Larry - K3PEG
Hello QRP fellows,
Thanks to all who sent me informations abt the GAP. It was extrem helpful to have this infos while experimenting.
Here are the results:
15m: no problem, adjusted the rod with the long ectention. Now resonant at 21.060 12m: no problem, adjusted the rod with the short extention. Now resonant at 24.980 17m: no problem, adjusted the shorter of the lower rods with an additional alu tube, resonant at 18.090 20m: The extraordinaire bandwith is a sum of resonant QRGs. I added a piece of alu tubing at the longer lower rod. 20 m resonant now at 14.070
10m: that was a little bit tricky. Adding some wire to the wire extention at the rod with the yellow cap had NO result. I added a piece of wire to the NON isolated aluminia tube oposite to the tube, where the short wire extention is connected )That one, with the yellow cap). My wire is abt 1 Meter long. This brought the 10 m resonance down to 28.100, the 40m resonance now was to low. I shortened the long counterpoise wire by 20cm. Now 40 m is resonant at 7.070 and 10m is resonant at 28.120
All this mods lowered my 30m resonance to 9.990. But because at 10.100 the SWR is less then 1.4, this is not a real problem. Only by means of interest I will try to add some cm of wire to the lower element of the GAP tomorrow morning.
The GAP Titan is resonant at ALL bands now allthough 30m is a little bit low. 20m bandwith is not so broad as it seemed to be first, but it is a multyresonance Problem.
72 and thanks again, Peter
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