Gizmotchy 2 Meter 4 Element Beam Review

[Marconi]

I had some time to do some tuning with the MFJ-259B analyzer tonight before it got dark. I mentioned during the assembly phase that I just took the gamma matches out of the box and attached them to see where it the match would be without adjusting them. I did this because I wanted to see how badly you could screw things up if you weren't reading or didn't understand that part of the manual. The out-of-the-box settings will still get you a match under 2:1 SWR on almost the entire 2 meter band, with only the extreme top of the band hitting the 2:1 SWR mark. However, the 1:1 SWR point was down around 135Mhz or so on both the horizontal and vertical segments.

I then set the vertical gamma match to the measurements indicated in the manual and tested again. This time, the SWR was well under 2:1 for the entire 2 meter band. The prescribed dimensions make the antenna usable with no further adjustment really necessary. I found the 2:1 bandwidth of the antenna to be 18Mhz, which is pretty broad-banded. Of course, I can't leave well enough alone (especially with an antenna analyzer in my hands) so I decided to try to fine tune the gamma match.

The manual's measurements still had my 1:1 SWR match point fairly low in the band, and I wanted it higher. Of course, this is just a matter of adjusting the gamma match's segments to find what works for you. For my desired frequency range, I found that pushing the second segment all the way down but leaving the top segment at the manual's dimension put me right where I was happy.

It got dark before I had a chance to adjust the horizontal gamma match, but I'm hoping I can have the horizontal side adjusted lower in the band to accommodate the typical 2m SSB frequencies.

More to follow...

Moleculo, nice review. It looks like the improved gain adjustment sent the bandwidth into the pits. Are you sure the final bandwidth at the factory setting was 18 mhz? The bandwidth for 2 meters is only 4 mhz wide,from 144 - 148.

 

[Moleculo]

Moleculo, nice review. It looks like the improved gain adjustment sent the bandwidth into the pits. Are you sure the final bandwidth at the factory setting was 18 mhz?

Pretty sure - I double checked it. The diameter of the elements and gamma match would make you think right away that it would be very broad-banded at 2 meters. Unfortunately, it is just too far below the ham band. What is interesting is that when I just installed the gamma matches out of the box without tuning them (at the default spacing), the match was also very low. Now after tweaking the spacing, I can't get a good match in the 2 meter band. This tells me that the antenna elements are inherently "long" and need to be trimmed.

 

[Marconi]

Pretty sure - I double checked it. The diameter of the elements and gamma match would make you think right away that it would be very broad-banded at 2 meters. Unfortunately, it is just too far below the ham band. What is interesting is that when I just installed the gamma matches out of the box without tuning them (at the default spacing), the match was also very low. Now after tweaking the spacing, I can't get a good match in the 2 meter band. This tells me that the antenna elements are inherently "long" and need to be trimmed.

I would be very cautious about trimming the elements. According to the 2 meter instruction you posted the boom length is a bit over 4' feet long. The 13" inch spacing for D1-D2 looks to me to be way too close, but I don't know for sure---I don't have any 2-meter stuff and never have had any, so I could be wrong. I think the boom should maybe be closer to 6' feet long for this beam and the spacing closer to being balanced across the boom.

The element lengths already look real close or maybe even a little short for 2 meters.

 

[Moleculo]

I had time to work on the Gizmotchy antenna project today and I made good progress!

I started by running the numbers of a standard 2 meter 3 element yagi, comparing the driven element length at various frequencies to this Gizmotchy. My hope was that by comparing the element lengths between 142 Mhz and 146 Mhz on a standard yagi, I could understand how much I had to shorten the Gizmotchy elements. When I ran the numbers, I also saw that the Gizmotchy elements were already a little shorter than the standard yagi. Admittedly, I don't completely understand the Gizmotchy, but I assume the shorter element length is due to the existence of the third leg on each element.

Running the numbers helped me understand how much I would need to shorten the elements by, but I still wanted to be cautious and not cut them down too much all at once. As a result, I decided to shorten them by 1/8" at a time and remeasure. Fortunately, I have a decent little bench top disc grinder which made quick work of the aluminum.

The end result was that the elements all needed to be shortened by 1/2" with the new, improved spacing. I wanted the low VSWR right around the 2m FM calling frequency, and here is the result:

The SWR is now under 2:1 for the entire 2 meter band, plus a little on either side .

After shortening all of the elements, I double checked the match on both the vertical and horizontal sides and raised the antenna up for some testing. Using the same fiberglass push-up mast, I raised it up to about 12 feet. Keep in mind that I'm on a hill above most of Los Angeles, about 800 feet above the valley below, so I don't need to get the antenna very high for it to be very effective. I hooked up my Yaesu 897D, put it on 2 meter and tested out my work.

The first thing I did was do some simple, typical yagi tests, checking out the side and rear rejection. I found some stations on the other side of the 6000 foot mountain that were barely breaking the squelch. With the side or rear of the antenna, they were unreadable. By pointing the antenna at the mountain, I could actually copy them, even though they were very scratchy. Turning the antenna around on other active frequencies, I was very happy with the side rejection and also found that the rear rejection was consistent with the revised element spacing's front to back ratio figures.

Next, I needed to test the effectiveness of switching between horizontal and vertical polarization, which is the whole point of this unique antenna. As anyone who has messed with VHF or higher bands knows, cross polarization of antennas will severely affect the strength of each stations' signal as you get into the higher frequencies. If the Gizmotchy is truly able to change polarity, it would exhibit these same characteristics.

The first FM station I found to test with was only about 5 miles away. His signal was pretty strong with the beam pointed at him, so I turned down the RF gain while horizontally polarized to get a measurable S meter reading of S9. I then switched the antenna to vertically polarized and he was 40 db over S9.

Next I wanted to test a station farther away. I found a distant repeater that I wasn't receiving nearly as strong and took the following reading on the Yaesu 897d while vertically polarized:

Note the S9 reading. Next I switched the antenna to horizontally polarized and measured again. This picture is of my radio while receiving the same station with the change in polarity:

The station was being received, but not even getting an S meter reading. Needless to stay, I was quite impressed and happy with the initial polarization tests! After the fairly simple mods to the antenna, this antenna is really looking like something very useful!

Next, the antenna will get mounted on the roof with a rotator and remote coax switch to make it easy to change polarization.

Stay tuned...

 

[HiDef]

Try using the AM position for relative S meter readings. Your radio's S meter does not do what you think it does on FM due to the I.F. limiter. The change between H and V pol should have shown about 17-20 dB. You got 40. Just about all hammy hambone FM rigs have this problem.

Thanks for posting. Great information for the next person who tries this.

 

[Captain Kilowatt]

Try using the AM position for relative S meter readings. Your radio's S meter does not do what you think it does on FM due to the I.F. limiter. The change between H and V pol should have shown about 17-20 dB. You got 40. Just about all hammy hambone FM rigs have this problem.

Thanks for posting. Great information for the next person who tries this.

I was going to post the same thing as I was reading Moles post but you got here before me. I learned that a long time ago when messing around with homemade 2m and 6m yagis. Signal strength changes on FM just do not tell the true story. It gets really bad when switching in a 10 dB preamp and seeing an apparent 30 dB increase in signal. :blink:

 

[Moleculo]

Not much AM 2 meter activity . However, I plan on doing the same test on SSB. Actually I tried to do that yesterday but I couldn't raise anyone.

 

[Captain Kilowatt]

You can monitor the output of an FM repeater while in AM mode if all you are interested in is relative signal strength. It simply sees the signal as a carrier and the modulation is not required.

 

[SR385]

I hope you forward all the numbers to the manufacturer....it looks like you really improved the product. Excellent writeup as usual.

 

[Moleculo]

I finally got the whole install on the roof and finished this last weekend. For reference, my QTH is on an 800 foot hill over looking most of the city below. As a result, I don't have to get a VHF antenna very high off of the roof to have tremendous results.

I had to move my 2m Diamond F22A dual cophased 7/8 wave vertical to another vent pipe mount to make room for the Gizmotchy. Here is a shot of the finished result:

The two antennas are not as close as they appear in that picture, although they are closer than is ideal. Here is another shot where you can see that the vertical's ground radials are actually installed higher than the Gizmotchy:

The reflector of the Gizmotchy is about 3' from the nearest vertical radiator when the Gizmotchy is pointed at it. It's not ideal, but the two antennas don't seem to care all that much...it is about 1/2 wave away, after all.

As you can see, the Gizmotchy 2 meter isn't really any bigger than a large TV antenna. It certainly doesn't weigh anymore than one, either.

For the rotator, I purchased an Ameritron AR-35 light weight unit. This is similar to the many others that have the same form factor. I didn't want to run three coax runs for the vertical antenna and the two Gizmotchy feed points, so I also purchased the Ameritron RCS-8V 5 port coax switch. This lets me change between all 3 "antennas" with only one home run feed line and a small control box. Here is a pic of the rotator and coax switch:

I've been doing some testing, comparing the beam to the vertical, as well as testing the dual polarization characteristics of the Gizmotchy. I'll report back as soon as I have some better numbers. Initial tests show that switching between vertical and horizontal polarization on this antenna produces very interesting results.

More to come...

 

[Marconi]

I finally got the whole install on the roof and finished this last weekend. For reference, my QTH is on an 800 foot hill over looking most of the city below. As a result, I don't have to get a VHF antenna very high off of the roof to have tremendous results.

The two antennas are not as close as they appear in that picture, although they are closer than is ideal. Here is another shot where you can see that the vertical's ground radials are actually installed higher than the Gizmotchy:



The reflector of the Gizmotchy is about 3' from the nearest vertical radiator when the Gizmotchy is pointed at it. It's not ideal, but the two antennas don't seem to care all that much...it is about 1/2 wave away, after all.

As you can see, the Gizmotchy 2 meter isn't really any bigger than a large TV antenna. It certainly doesn't weigh anymore than one, either.

I've been doing some testing, comparing the beam to the vertical, as well as testing the dual polarization characteristics of the Gizmotchy. I'll report back as soon as I have some better numbers. Initial tests show that switching between vertical and horizontal polarization on this antenna produces very interesting results.

More to come...

Moleculo, could you tell us what the spacing differences are between the antenna above as compared to the antenna in the instruction manual you posted below?

http://www.worldwidedx.com/attachme...ter-4-element-beam-review-2m_antenna_inst.pdf

 

[Moleculo]

Yes, the spacing on mine is exactly reversed from what the diagram shows. So, the Reflector to Driven is 13", to the first Director is 18", the second Director is 21".

BTW, I've discussed these changes with Tom Charles, the owner of Gizmotchy and they're going to incorporate these changes immediately. I encouraged them to validate my findings and do their own models to verify the findings, so it will be interesting to see what happens.

So far my testing indicates that the antenna performs consistently with the revised model, but I still need much more use to be completely satisfied. The front/back ratio seems like it is dead nuts on what the new EZNEC model showed based on the S meter readings. The deep null shown on the model is certainly present. I can turn the antenna so that a distant station is right where that null should be, and sure enough, that station goes away.

The vertical antenna is a Diamond F22A, which is advertised as 6.7 dbi gain. The revised Gizmotchy modeled at 9.16, which is about 2.5 db more than the vertical. Switching back and forth between the two vertical antennas shows me that real life testing indicates that the increase in gain on the beam is realized. Depending on your S meter, 2.5 db is about 1/2 an S unit, and my Icom 746 Pro shows this to be the case.

Switching between horizontal and vertical polarity has also proven to have some very interesting results. I tested with a fairly strong station about 27 miles away, first monitoring on AM (he was using FM) and switching between the two polarization methods. This yielded a solid 2 S unit difference, with the horizontal polarization dropping him down to an S5 from an S7. On FM, it's another story completely. I then tested with another station on FM who was about 29 miles away, a little weak, and using a discone antenna that wasn't very high. With the beam directed right on him, I was able to bring him up to about S7. While switching to horizontal on FM, he dropped down to almost no copy, barely S1.

I haven't had the opportunity to work someone on horizontal polarity SSB yet, so I can't report those results. I did work a station who was vertical on 2m SSB who was about 15 miles away. Switching the Gizmotchy antenna between polarities made the difference between an S9 signal and almost no copy. The other station reported the same results, as well.

It's definitely interesting how much difference cross polarization can affect things at VHF, especially when using FM and SSB.

 

[Moleculo]

Here is an interesting problem that I had to troubleshoot over the last couple of days: When I put the antenna in horizontal polarization and pointed it directly east, I got horrible RF feedback in my rig. It was enough to garble the TX audio on FM or SSB, and you could definitely hear the problem while monitoring with headphones. If I switched the antenna to vertical polarization, the problem wasn't there at all. Very weird. Then I realized that the antenna is pointed directly at me when it's straight east.

I started troubleshooting by checking RF grounds on equipment, but I couldn't find anything unusual. I thought maybe it had something to do with all of the audio or mic equipment, but it didn't seem to matter what I switched on or off. So then I decided to change back to the stock mic to eliminate the audio gear as a possibility. Well guess what...the problem went away.

So now I knew that something in the audio chain was acting like an antenna and picking up the RF when the antenna was pointed at me. I plugged the Heil mic back in that is attached to all of the W2IHY gear and did some more tests. Keep in mind that the mic is attached to a boom that extends out over the radio equipment to the front of the desk. When the boom is extended a little, it is sort-of an upside down V shape, with the broadside pointing east and west in the same plane as a horizontally polarized antenna pointing east would be. So, I thought...hmmm....maybe the boom and mic cable are acting like the antenna and picking up the RF. So I key down the mic, listening to all the garbage TX through headphones, and swing the boom around so it is not 90 degrees to the plane of the 2 meter signal. Guess what...the RFI goes away and the TX clears up. So now I really know what the culprit is! I checked the grounding of the mic cable and equipment and still can't find anything to fault. So, I pull out my box of ferrite torroids and snap one on the mic cable right by the mic and start adding them at the base of the cable on the other side where the boom ends. After adding several on the other side of the cable, the RFI in the TX goes away. The mic cable was indeed acting as an antenna when it was stretched with broadside east/west and the antenna pointing east in horizontal polarization and the ferrite got rid of the RF!

I imagine this problem would go away if the antenna was mounted higher. If you look at the 3D EZNEC model, you can understand why this might happen. It's also kind've funny that it only happened with the exact combination of equipment, equipment layout, antenna, and RF polarization. Isn't amateur radio fun!

 

[Captain Kilowatt]

Chasing down RFI problems is a load of laughs isn't it? :headbang

 

[Ronin]

Any new information on the Gizmotchy 2M 4Element? Were you ever able to give the horizontal side a run on 2M SSB?

I have been seriously considering one since I temporarily live in a rental home and liked the "lower profile" aspect of the design. At this point, I don't really need high gain figures, but would like to be able to put a little more signal in one direction as well as gaining the horizontal aspects for 2M SSB.

Excellent review by the way! Thanks!