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Review of the 30 Meter MonoGAP

Posted by on October 16, 2014

Fully assembled 30 Meter MonoGap antennaThis review of the 30 Meter Mono GAP, (see my review of the GAP Challenger), was prompted as a first phase for a test bed for constructing a 30 Meter 4-Square phased array.  I also wanted to get a decent antenna for 30 Meters, and I like vertical dipoles because one does not need the huge radial field a  monopole does.  In fact the GAP 30 Mono has only three radials, which aren’t really radials, they are more of a capacity hat for tuning.  The GAP 30 seemed a good selection for a 4 square system, as I would not need to lay out miles of radials, the antenna is small and need no guying, 30 Meters is a fun band, and will be active across Sunspot minimum times, and I have always wanted a vertical phased array.  So I decided to buy one, and test it.  This review is the result of the install.  Look for addendums regarding performance at the end of this article later…  For a closer look at any photo, just click it…  Once you have finished inspecting it, hit the BACK button on your browser.


30 Meter MonoGap parts as they arrived, and out of the box

Out of the box…

The antenna was ordered via HRO, as I found it cheaper to order from them, as opposed to ordering directly from GAP.  HRO had less postage involved.  So the first tip…  Look at the cost of postage…  The antenna took a while to get here, as HRO did not have one in stock, and had to order via from GAP.  After about a two week wait, the antenna arrived…  Again it looks as if the shipper has beaten another package up…  Note the foot print on the box in the above photo!  Fortunately the packing GAP provided protected the antenna from the shipper…  I doubt GAP shipped it this way…  The shipper was UPS!

Most of the parts from the antenna dropped right out of the box when I upended it to get everything out.  The instructions for assembly however did not, so I assumed there were none, this somewhat ticked me off at GAP…  Only upon breaking down the box for the trash did I find the instructions.  I had already downloaded the instructions from the GAP site and had finished building the antenna.  I am sure the instructions hung up on the big tear that was in the box when I upended it to dump part out, so it was not GAPs fault!  After laying out all the parts I decided to do the inventory one should do each time one assembles an antenna from a shipment box…


30 Meter MonoGap main mast, showing parts label

Named Items

At this time I also decided to write the name of each part on each part, (see photo at left), this practice always makes it simpler to construct an antenna outside, if each part is named what the instructions call them…  That way I am not sorting out what part is what, outside in the heat, or rain…  This is also the time I organize all loose hardware, like screws and nuts in an egg carton for assembly later.  I have found that taking a bit of time in the house, where the temperature is comfortable, going over the instructions has always helped in antenna construction.

That’s when a bit of trouble began with regards to the instructions not being clear enough for me…  If you look at the left most element in the photo captioned “Out of the box”, above and to the left here, you will see the folded separators for one of the tuning elements, this antenna section is called the “Lower Section” in the instructions.  One is supposed to fold out the long plastic tubes so that the tuning stub can be dropped into pre-drilled holes in the plastic rods you just folded out.  That is simple, what was not simple was deciding which end of the lower section should be pointing at the sky.  It will make a difference in tuning, as the instructions show the tube as having the plastic mounted asymmetrically…  The instructions clearly show the “Lower Section” as not having the tuning rod holders mounted symmetrically, while the actual physical Lower Section has them mounted symmetrically out of the box.  Off to call GAP…  It turns out that for the 30 Meter antenna, they ARE mounted symmetrically, and the instructions are in error…  That out of the way, I was able to label the tube as “Lower Section”, and add an arrow pointing towards the sky on it.

Pentrox as used on the 30 Meter MonoGap


While on the phone to GAP I mentioned I was using Pentrox to keep things clean over the years, and they indicated they do not recommend the use of Pentrox.   Something about some people having better results without it…  I ignored this warning, and used Pentrox anyway.  So far I have had no Pentrox related issues.  I have used Pentrox for decades to keep Aluminium parts from locking together and have no intention of changing now…  Perhaps others are slathering on way too much Pentrox, I used just a little and then use a paper towel to spread it around…  Remember, if you use Pentrox, the manufacture DOES NOT recommend it.  Also NEVER EVER use steel wool on the antenna tubing…  If you do, you will be punished in a year or so…  Turns out the steel wool which is– well– steel, gets between the aluminum tubing sections then starts to rust…  So NEVER EVER use steel wool on any antenna…

This brings us to another point of interest…  The supplied PL-259 connector does not look like a Silver/PTFE connector…  I use Silver/PTFE on ALL coax connections…  My goal is to put it on once, and never have to mess with it again…  So I replaced what looked like a non silver/PTFE connector with a nice new Silver/PTFE connector from DX Engineering.

Which now brings us to the feedline used by GAP.  Make really sure your knife is very sharp, as the center insulation of whatever feedline is being used is a really, really hard plastic, and very difficult to cut with a sharp knife, much less a dull one.  I expected trouble when I put on the connector, but none ensued…  I would like to think it is my masterful process for putting on a coax connector, but I doubt it.  I think I just got lucky!

30 Meter MonoGap fully assembled, and by side of home

I chose to assemble the antenna along the side of the house.  I have about a 50 foot long, by 15 feet wide gravel driveway and it makes for a good place to build verticals…  I stood one saw horse up, to keep the top off the ground, as I assembled the antenna.  The antenna went together well up to the base section…  GAP has thought of the erection phase of this antenna, and provided a small nipple sticking out of one end of the base section made from the next smaller sized tubing section.  This is to allow one to insert the smaller pipe into the supplied insulating tube for final mounting…  The only problem is the instructions don’t say a single thing about this, the builder is left wondering why the tubing called Base section has non symmetrical ends, and which end goes where…  Off to call GAP again, they explained that the nipple side of things goes down, and the open end of the tube goes up…    GAP does this to make it simpler to stand the antenna in the mounting pole, and it is a lot easier to put up because of this, it just would have been nice to know what it was all about during the construction phase of things…  GAP really needs to re-write the instructions.  I will be happy to if only GAP would send me one of each antenna…


Pentrox on Screw on main mast of 30 Meter MonoGap

Pentrox on Screw

Beyond the items mentioned here, all went according to plan.  I used Pentrox between each slip joint, and on each screw connection.  With the antenna finished, I removed my second GAP Challenger, and then disassembled it.  I love Pentrox, the antenna came apart like butter. nothing seized up, nothing bound, the entire antenna just slipped apart…  That is why I use Pentrox on EVERYTHING Aluminum for all antenna work.  I have no idea why GAP is getting reports of problems when Pentrox is used.  I have used it for decades on antennae, and never had an issue.  I suspect the Pentrox is somehow being misused…

I next placed the GAP 30 Meter Monoband into the old GAP Challenger mounting pole.  Inserting the 30 Meter Mono GAP was simple– remember that large nipple GAP added, it actually helps!

The GAP 30 Meter Mono bander is MUCH shorter than the Challenger it replaced, so off came the guy lines for the challenger, as I have two Challengers up, this tended to help clean up the look of the back yard a bit as well.  I also removed the old Challenger ground mounted capacity hat at the same time.  This left the GAP 30 Meter monobander sitting alone on the mount, with no guy lines, and no ground radials.

I connected the 259B analyzer and had no dip anywhere close to 10 MHz…  I looked over the antenna, and noticed I had connected the top tuning rod, to the lower section of the antenna at the gap, not the top section as the instructions indicated…  I pulled the antenna down, and corrected my error…  Reconnecting the 259B I noticed a nice dip at 9.7 MHz.  At 9.7 I had a 1:1 SWR.  I suspect I need to trim the radials a bit to raise the frequency…  A quick call to GAP confirmed this, and they instructed me to trim at 6 inch intervals…  They also indicated that I would end up with a 1.5:1 SWR across the entire band.

I ran outside, and clipped off six inches of radial…  Upon coming inside, I noticed the resonant frequency went down, not up…  I chalked that off to measurement errors on my part, so I removed another six inches of radial from all three radials, and the resonant frequency again went down again!  OK I now know it is not me…  I decided to leave the problem for next summer.  I do not want to get into a muddy mess in Oregon rain dealing with the antenna, and it is performing so well now…


Overview of Lot, showing ground radials for various antenna

Scaled view of project

The test bed is as follows:  All antennae are set up in a large empty field, perhaps 300 long by 70 feet wide.  There are six antenna mount points in the field, all fed by underground 4 inch PVC for cables.  Four are to be used for a 30 Meter 4-Square array later, and the remaining two for a two element 40 meter array.  There is another antenna mount point on the rear of the house for a dipole I use for reference.  Remember, all tests are relative to my dipole…  I do not have the gear to make absolute measurements unfortunately.

This phase of the testing is to verify that the GAP 30 Monoband will perform well, and if it will phase up.  I performed a similar test for the GAP Challenger, using mount points 5 and 6, only to discover, (as expected), that phasing a multiband antenna is not a simple task.  While I could arrive at good phase, I could not control the RF amplitude at each antenna well enough to make it work, so I abandoned the idea of using GAP Challengers as elements in a phase array.  I did not really expect them to work, but given I had two Challengers, I decided to give it a try anyway…  As expected the mutual coupling between antennae proved to be too great because the Challenger is a multiband antenna, and having all those other bands interacting is just too much to keep track of…  So positions 5 and 6 will end up in 40 meter mono gaps, (they are spaced for that anyway), someday, and the GAP Challenger will move to a new position someplace else in the yard.

30 Meter MonoGap mounted to vertical post in cement, shows ground radials, and lighting choke

Antenna mount point #5

This is one reason I use GAP antennae for most of my testing, I do not need to dig up a monster radial field for each move…  Because the GAPs are vertical dipoles, they do not need the radial field a vertical monopole must have to work correctly.

I am now using mount point 5 as the test location for the GAP 30 Monoband antenna.  The GAP 30 Monoband is ground mounted, with the requisite number of radials as specified in the instructions.  The dipole at about 1/4 wave height on 30 Meters, and coming off the house, and is used as the comparison antenna.  It is about 2 DB down from the Challenger.  The Dipole is also a BWD-90, which has about 3 db loss over a normal dipole.  Both antennae are fed with new, quality feedline, (RG-8X).  The GAP is on about one hundred total feet of feedline, 50 of which is DX Engineering MAX400, and the remaining fifty feet being RG-8X.  The GAP also is connected to a Ameritron RCS-4, at that 50 foot mark, while the Dipole is run directly into the back of the Elecraft, with nothing in between it and the rig but two PL-259s.


30 Meter coverage map of 30 Meter MonoGap antenna

30 Meters coverage of GAP 30, using 1 watt from Oregon

The first thing I did was to start WSPR off using the new antenna…  All the usual suspects came in all a bit better than before!  Next I decided to see what one watt could do using JT65 and PSK Reporter…  The photo at the left shows what happened, I am transmitting from Oregon…  Remember, this was using only one watt, during the evening…  I have tried this many times using the test dipole, and never gotten results this good!  The GAP performs much better than the dipole ever did…  I ran a test with a local ham, using the dipole, and the GAP.  Both came out the same for signal strength on his system.  He lives about six miles from me.  We ran the test at low power, so my signal at his location was in the S5 to S7 range.  I am now doing comparisons to stations further out, and/or DX stations, and switching between the GAP and the dipole in rapid succession.  The GAP Monoband seems to be at least one S unit better on mid US stations than the Dipole, and maybe two on DX stations.  The GAP Monobander is noisier that the dipole, but that is to be expected.

GAP 30 Meter MonoGap mounted with the Gap Challenger in place behind it

GAP 30 Meter Monoband in place

I have now had the antenna up and running for about a week…  I am still using a tuner, because I am unable to resolve the too low in frequency resonance issue, and the inability of the antenna to move upwards in frequency by shortening the radials, as per the manufacture…  I have a GAP Challenger up 33 feet away from the GAP 30 Monoband and this may be related.  I will take this issue up once summer 2015 begins, when I can work on dry ground, not wet Oregon winter soil.

My overall feelings towards the antenna– it seems better than the test dipole in all cases.  I can A/B the two by simply flipping a switch on my tuner, (this insures both antennae are tuned correctly), and in all cases, but local hams, the GAP 30 Monoband is better by at least an S unit, and in most cases two S units.  Also in most cases prior to installing the GAP 30 Monoband my reports were always less than the other stations report.  i.e. The contacted station would get -10, and I would get -18.  Now, I am seeing parity, or better most of the time.  That is a significant change.  That I believe is the most important metric here.  Conditions change, my switching speed between two antennas change, but seeing my signal reach parity more of the time than with the dipole, (at the same power levels), tells me the GAP is doing better than the dipole is doing.  Using the GAP 30 Monoband antenna QSB hits a bit harder, but the signal is higher to start with, as opposed to the dipole.  Overall I am sure the GAP 30 Meter Monoband antenna is better than the dipole for DX, and that is all I really care about…


Yes, I would recommend this antenna to a friend.  It seems overall a better antenna than I have ever had for 30 Meter DX.  I am trying to build my station for the future, when there are no more Sunspots, and all I have are 80/40/30 meters to play with…I have room for two more antennae on the RCS-4, and will figure out what I need by summer 2015, and install them then…  All in all, I am happy I bought this antenna, and if it holds up like the Challenger DX I own, I am sure I will get many years of service out of it.  I did not have to guy the antenna, but we almost never have high winds, and the antenna is light weight enough one person can take it down, and put it back up.  The cost is good, it only cost me 179.95, plus 16 or so bucks postage.  I sold the second GAP Challenger to fund this one, and it was well worth it!  This leaves me with a single GAP Challenger, a single GAP 30 MONO, and my dipole…  I did couple this project with another one for getting my antenna feedpoints away from the house, and closer to the tree you see in the photo on the left.  I expect there will be an 80 Meter dipole or a AlphaDelta DX-CC there in summer 2015.  Look for the review then…




Reasonably high winds arrived in the Valley here, perhaps 35 to 40 MPH…  The 30 Meter MonoGAP held up fine…  It flexes a lot, perhaps 25 to 30 degrees bend in it at the top, but no damage of any kind.  I think I will add guy lines anyway this summer, but it did hold up fine.



I have been able to get very good performance on 15 meters with this antenna and a tuner…  Better than my BWD-90 by far…



I have had a long time to test the GAP 30 Monobander, and am happy to report it is a very nice antenna for DX.  SWR is a bit high on it, but I can live with that, and the antenna performs as expected for a vertical.  Like all verticals, it does pick up more noise, but it also picks up more signal from DX locations than my 30 Meter dipole at 25 feet does.  I will be ordering a second one this summer, (2016), and in late summer, move the two antenna to positions 3 and 4 of the antenna farm in the back yard.



It has been two years now, and no issues of any kind with this antenna.  Works like a champ on 30.


12 Responses to Review of the 30 Meter MonoGAP

  1. Jerome Borucki

    how long is the antenna from top to bottom?
    Jerry W6GSJ

  2. Robert Montgomery

    Great review.

    I’m getting ready to purchase a Gap Mono 30 in the next week or so and your review helped to seal the deal for me.

    Did you ever arrive at a resolution regarding trimming the counterpoise?

    Robert, K6FNI

    • Dave

      Hi Robert,
      I am still at 2.5:1. I suspect the SWR can be brought lower, and I will try a bit more this summer, but the antenna works so well, I may just use the tuner on it. Glad you will be getting on 30, it is blast of a band!

      • John

        When I add radials to my ground mounted 1/4 wave vertical antenna the resonant frequency goes up. This is a result of the increased capacitance between the antenna and the radial system. The increased capacitance nulls out some of the inductance and that acts to electrically shorten the antenna and thus raise the RESONANT frequency. I compensate for this rise in frequency by physically making the radiator element longer.
        You’re using a vertical dipole and no radials are required. I don’t understand why GAP tells you to install radials. Reducing the radial length should decrease the capacitance between the radiating element and ground and thus lower the resonant frequency. Does GAP provide some type of device to match the antenna to your transmission line?

        • Dave

          The GAP uses the radials to add capacity, not as a grounding system. I believe they act like a capacity hat only on the ground, not in the air. GAP provides nothing for matching. I have been looking for a model I can dump into a modeling program, but alas, I may end up building my own. Someday when I have the time, I will develop the model, and then make it available…

  3. Lawrence

    [ cannot get my gap challenger to work on two meters.

  4. Dave

    Hi Scott,
    I tried a single feedline, and all results were the same… Good thought though! I also added a line in the dipole description which answers your question, the dipole is a BWD-90, which is about 3 db down from a normal dipole on that frequency. For some reason I just assumed everyone knew what I used for a dipole! 🙂

  5. Scott Currier

    Do you think there is any chance that the slight impedance difference between the two pieces of coax is causing your problem with the resonance being lower than expected?

    I’d almost be tempted to try a single run and see if it makes any difference.

    Nice review. BTW, I just want to confirm that you’re comparing this antenna to a 1/2 wave center fed dipole at 1/2 wave above the ground. That’s about 50 feet. I just find it hard to believe that the vertical is better.

    If that’s the case, maybe I’ll hang a 30 meter dipole from a tree and see how it plays against my 30/17/12 meter fan dipole.

    73 Scott KT1B Haverhill, MA

  6. Alex RK6C

    Good job! Congrats with new antenna! GL and many DX!

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