Gizmotchy 2 Meter 4 Element Beam Review

[Moleculo]

Thanks for your input!

With regards to this:

In reference to a previous post on RFI to w2ihy equipment, I found that above 10m the ihy box is not RFI filtered well enough at 6, 2 and 70cm.

The RFI wasn't actually entering into the W2IHY gear. It was coming through the mic cable on my Heil boom that has the same exact angle as the Giz when the beam is pointing directly toward the operating position. Some ferrite snap on beads over the cable on either end solved it.

 

[KJ4VOV]

If you're going to buy one, it might be worth a call asking if they have made those changes yet. If not, at least I've shown you what to do .

Just as an FYI, I bought one last Sunday at a hamfest and the instructions showed the old spacing.

Of course, judging from the condition of the box this was likely NOS, kicking around the dealer's warehouse for a few years. The copyright date on the illustration is 2006, but the written instructions are dated 2007.

 

[KJ4VOV]

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...

I'm a bit confused by this Mole. First off, my gamma matches didn't come assembled and, second, the four page "manual" that came with mine doesn't even show the full vertical gamma match, just the tip of it. There's a fine picture of the horizontal that shows all the dimensions, but nothing showing the dimensions of the vertical. Looking elsewhere in the manual I find a section detailing assembly of the gamma matches, but it states that the overall length is the same (4.75") on both. I have a feeling the manual changed between when my antenna was made and when yours was made. I will have to borrow my buddy's MFJ-259b and do some experimenting. Unless you have the dimensions of yours listed somewhere?

Also, I was over on the Gizmotchy website the other day and I see they now sell a cable set to allow circular polarization, which I may have to try making on my own.

 

[KJ4VOV]

The distance between elements is a follows
reflector to driven 21"
Driven to director 1 19"
director 1 to director 2 12"

Modeling this antenna if you change the spacings to:
Ref to Driven 15 1/2"
driven to dir 1 12 11/16"
Dir 1 ti dir 2 20"

The forward gain will be improved by 1/2 dB, however the F/B will improve by 4 dB

Depending on the amount of activity around you the front to back improvement may or may not be worth the effort. The total spacing is reduced to about 48"

Excellent info! Did you run the model with the element lengths 1/2" shorter where I ended up at?

Also, can you post a picture of the new model you ran?

I too would love to see the new model because that 4db F/B improvement is impressive and I'd like to see what the pattern for the side lobes looks like now.

 

[Moleculo]

Unless you have the dimensions of yours listed somewhere?

I might have the gamma match dimensions on my PC. I'll poke around and see if I can find them.

 

[KJ4VOV]

I might have the gamma match dimensions on my PC. I'll poke around and see if I can find them.

Any luck?

 

[Moleculo]

Any luck?

I've looked everywhere, both in my paper files and electronic and I can't find the installation instructions. Have you tried emailing Gizmotchy? I'm sure they have them to email you; they emailed me all of the initial antenna models.

 

[KJ4VOV]

Thanks Mole, but I think we have some miscommunication here. What I was trying to do is save myself multiple trips up and down the ladder tuning the gamma matches. I have a bad knee and after about the 4th trip it starts screaming at me. What I was asking for were the final dimensions you used on your gamma matches after tuning the antenna. My manual lists them both the same at 4.75"

Sorry if I confused you.

Also, has anyone tried the dimensions W4JLE posted, or seen his models? Do we know if he was using the shortened elements or not? (I may have to email him...)

 

[Robb]

Thanks Mole, but I think we have some miscommunication here. What I was trying to do is save myself multiple trips up and down the ladder tuning the gamma matches. I have a bad knee and after about the 4th trip it starts screaming at me. What I was asking for were the final dimensions you used on your gamma matches after tuning the antenna. My manual lists them both the same at 4.75"

Sorry if I confused you.

Also, has anyone tried the dimensions W4JLE posted, or seen his models? Do we know if he was using the shortened elements or not? (I may have to email him...)

I've found it much easier and just as accurate to adjust the gamma match with the antenna resting on its reflector on the ground and the beam pointed skyward. Once you get it set up correctly and adjust it, just walk the beam back up the ladder and install.

Much easier this way - IMO . . .

 

[KJ4VOV]

Just finished swapping the SO-239 connectors on the gamma match plates to right angle SO-239. That eliminates needing an adapter to connect the coax. One less tiny bit of loss, two less things to weatherproof or worry about loosening. Tomorrow I'll see if I can borrow a buddy's MFJ-269 and start the tuning. I still don't know if the latest spacings are with the shortened elements though, so I'll try them first with the uncut elements and see how it looks.

I considered using N connectors, because they're better connectors, but 90% of my coax cables have PL-259s on both ends, so I'd still need adapters.

If all goes well and I get it tuned up I plan to give it a good field test on Sunday. My local club is supporting a "Lakes & Grapes" 100 mile bike ride to several local wineries on Sept 8th and this Sunday my task is to go to each of the rest areas along their route and see what conditions are like contacting another station at the Start/Finish line where our NCS will probably be. The longest straight line distance will be just over 26 miles, but with the rest area in a river valley and a couple of hills in the way I think it's going to be very "iffy", so the beam will help. I'll try this beam mast mounted on the truck at an elevation of about 24'. If I need to go higher I have 48' of mast I can use, but I'll have to get some locals to help with the guys.

(If needed I also have an Elk 144/440 5 element log I can use instead)

 

[KJ4VOV]

Still trying to get this tuned with the most recent spacings. Got it down to 1.5 @ 146.520, but 2:1 towards the band edges (only doing vertical so far). A friend suggested that I'd want to put a dummy load on the side not being tuned and that made so much sense to me I went out and bought a small one to use just for this tuning. I'll get it on the MFJ-269 again tomorrow and see what happens. When I do get it tuned I'll post the exact measurements of the gamma matches.

 

[KJ4VOV]

Well, after playing with it and playing with it I simply could not get it to tune properly, so I took it to a friend's shack and we hooked it up to his full blown analyzer. The results were "interesting". The ability to graphically see the SWR across the entire band (and beyond) as a plotted waveform made the task a bit easier, and showed exactly what result each adjustment got us. Starting with the most recent spacing info posted by W4JLE we found we could not tune it for less than 1.6:1, and then only for a small portion of the band, either on vertical or horizontal. After an hour of trying we gave up and went to the spacing suggested by DXER. Again, while better, we could not get SWR down below 1.5 no matter how the gamma matches were adjusted, and the waveform display was looking rather strange, with a sharp increase in SWR around the center of the band and leveling off at the low and high ends. At this point my buddy AK1E commented that with the two spacings we'd tried it would make a heck of an aircraft band antenna, nice flat SWR across the entire aircraft band, it was not a very good 2m antenna.

Then, after 6 hours of adjustments and measurements, just for the heck of it, we decided to try the stock configuration, as listed in the assembly instructions. BAM!

Perfect SWR across the entire 2m band (1.2:1 or less) on both horizontal and vertical. That's with both gamma matches set exactly as shown in the manual and with the "reversed" element spacing the manual shows.

I'll be doing some field testing with it this coming week and we'll see how it works.

 

[KA3YAN]

Bernhard,

Was wondering how you fared in further testing? I haven't had the chance to put the antenna together yet, but I'm very curious about your results. Strange that we've had two separate NEC models that show one thing, but in your real world tests, it shows something completely different.

73,
Brad KA3YAN

 

[KJ4VOV]

Sad to say, I've not done anything further with this antenna. For awhile I carried it in my truck, thinking to use it for some of the public service events our local ARC participates in, but it took up too much room and got moved out. Then I considered rotator mounting it on the house, but the controller on my rotator died and I've not gotten another yet. So right now, it's sitting in my garage on top of my generator.

 

[KA3YAN]

Hahaha. No worries. I have a few projects that are in exactly the same state. Well, I think I'll attempt to get the antenna tuned and erected this coming weekend. It was a gift from one of the officer's of my state's Navy & Marine Corps MARS. I plan on using it on a Packet RMS gateway for Winlink.