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Gizmotchy 2 Meter 4 Element Beam Review
- Thread starter Moleculo
- Start date
This one pretty much tells the story of the specs on the antenna:
It is interesting to me that the gain of the antenna appears to be about the same as a typical 3 element yagi, although the front/back ratio doesn't appear to be as good as expected.
I certainly would like to hear what everyone has to say about these models.
I'm not real impressed with the shape of that pattern or with the forward gain figure of only 7.88 dBi. That's only about 5.5 dB over a dipole and a four element should do a couple dB more than that. I had a homebrew four element quad that had a MUCH better F/B ratio than that and while I could not measure the true gain I am certain it had more than that. It would be interesting to see he results of adjusting the element spacing to achieve a better F/B. I'll bet the gain would increase a bit too at the expense of bandwidth. With a dual polarity antenna bandwidth should not be a concern,tune the vertical for the high end of the band and the horizontal for the low end where all the SSB activity is.
I've attached the installation diagram that has the antenna's dimensions. If someone skilled with EZNEC wants to load this data up and play around with the element spacing to see if they can improve the front/back ratio and gain, that would be cool. Then I could try out the new settings after testing the stock antenna.
Well that leaves me out. I never could understand why they called it EZnec because I never found anything EZ about it. Maybe I just need more kidfree time to try it.
As a side note the Cushcraft 3 element A-148-3S has a claimed gain of 7.8 dBd note the dBd. I find Cushcraft's gain figures to be pretty much in line with what they should be on all their antennas.
read my 1st post in this thread. as i said, i was just a kid way back then, but even as a kid i wasn't sure what the infatuation with 'gizmotchy' was. i found numerous other homemade & commercially manufactured antennas to be better performance-wise then the 4 & 6 element 11m gizmotchy's i had a hand in. and while they aren't 'jo-gunn priced', they ain't cheap either. i still wanna HEAR what moleculo thinks about these things, but based on above....i'm not liking it. still.
I'm certainly no expert with EZNEC, but I have been playing around with the demo version of the software that is free to download for a couple of months now. It is not as EZ to use as the name suggests, but I have modeled various dipoles, quads and yagi's and been able to achieve the results that you would expect to find from the antenna I have modeled. One thing I have noticed, is that modeling any antenna in free space can be quite misleading. There are options in the software to model over "real ground" where you can input the characteristics of the soil that you actually have in your back yard (if you know what that is) or a "perfect ground" option. When using the real or perfect ground settings I get the gain figures that I would expect, if modeled in free space the gain figures are substantially lower. Just modeling the gizmotchy over real ground rather than free space would increase the gain figures to something like what you would expect to see from a 4 element antenna.
Once a 2m antenna is a few wavelengths above ground it is essentially in freespace. Ground conductivity has little or nothing to do with pattern shaping on VHF and above. HF is a different matter altogether.
I didn't think of that, lol. I have only ever used the software on HF antenna's. Carry on,
You're reading the NEC plots correctly, it's a bit more focused than a two element beam or a short quad. Better than not having directional ability, but far from surgical.
That has big advantages if you have a group of people or a mobile you are tracking, the wider beam width makes it easier to reach the group or a moving mobile.
That has big advantages if you have a group of people or a mobile you are tracking, the wider beam width makes it easier to reach the group or a moving mobile.
Finishing the antenna installation and review has been delayed a bit, mainly due to weather. It keeps raining on the weekends! The other thing that has delayed me is that everywhere I have gone to purchase right-angle PL-259/SO-239 adapters has been out of stock. You need these to feed the antenna because the element that slopes down is in the way of the SO-239s. You can't get a standard PL-259 in there because it hits the element. So, I'm trekking around trying to locate parts. Hopefully this coming weekend will have better weather so I can get this done.
Hello to the group. After going over this design with Shoemaker we decided to homebrew an eznec file and take a look. I believe someone made a mistake when the instruction manual was drawn up for this beam. My nec file has the elements at a different degree angle than the previous posted nec results but the results were very close. After standing back for a second and looking at the antenna it seems as if the spacings along the boom were backwards. Starting from the reflector out the spacings should be 13,18,21
placing the reflector at 0, driven at 13, 1st dir at 31 and 2nd dir at 52. The beam is still low on the band but all elements can be scaled easily to come up in freq. Here are the plots I came up with. Let me know what you think.
View attachment 2m-2.bmp
View attachment 2m-1.bmp
View attachment 2m-4.bmp
View attachment 2m-3.bmp
placing the reflector at 0, driven at 13, 1st dir at 31 and 2nd dir at 52. The beam is still low on the band but all elements can be scaled easily to come up in freq. Here are the plots I came up with. Let me know what you think.
View attachment 2m-2.bmp
View attachment 2m-1.bmp
View attachment 2m-4.bmp
View attachment 2m-3.bmp
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Don't forget to mention that in the review! That is something the company should have taken into account and either changed the design or included an adapter with the antenna.
What I think is that if you're right, you just made an outstanding improvement on this antenna! This is my summary of the key results by adjusting the spacing:
Old spacing results
Beam width: 60.7 degrees
Front/Back ratio: 8.18 db
Front/Side ratio: 8.18 db
Gain: 7.88 dbi
New spacing results
Beam width: 58.3 degrees
Front/Back ratio: 19.03 db
Front/Side ratio: 16.45 db
Gain: 9.16 dbi
Your spacing changes resulted on a little over 1 db gain, but more importantly improved the front/back ratio by almost 11 db!! The front/side ratio improved by over 8db!!
Thank you very much for doing that analysis. After thinking about it, I agree with your findings that the dimensions you indicated are much more typical with what you would expect a beam to be.
I'm going to adjust the spacing on mine and see what happens.
HELLO MOLECULO HOPE EVERYHTING IS WORKIN OUT WITH THE ANTENNA. WHEN DXER AND MYSELF WERE MODELING THE ANTENNA WE NOTICE THAT THERE WAS A WIDE SPACE BETWEEN THE REFLECTOR AND THE DRIVEN ELEMENT, IT SEEMED REAL ODD. SO WE MODELED IT THE OPPOSITE WAY AND "BOOM". I WAS AMAZED AT THE ERROR(BUT NO ONE IS PERFECT). HOPE YOU HAVE FUN WITH THE ANTENNA AND GOOD DXIN.
73'S SHOEMAKER.
73'S SHOEMAKER.
We finally had a nice weekend here with no rain!
I spent some time today re-spacing the elements according to the new EZNEC dimensions. This didn't take long because in reality, only the two middle elements needed to move. As it turns out, the SWR ended up being exactly what the new EZNEC model predicted: The best I could get it was at about 142-143 Mhz. It would remain under 2:1 at the low end of 144Mhz, but I couldn't get it up into the middle of the band, regardless of how many different ways I adjusted the gamma match. It looks like I will have to trim all of the elements a bit.
The "Review the Gizmotchy" project has turned into a "How to Improve the Gizmotchy" project If I get it right, the front/back ratio improvement will be worth it!
I discussed in an earlier part of this review that I had some concern over the stainless steel hardware that is included. As I mentioned, it has been raining a lot here. Here is a close-up picture of what the included bolts and nuts look like now:
The U-bolts and hose clamps are all good quality stainless steel. The one bolt I had to replace is also rust-free. Before this antenna finds it's finally install location, the few bolts that have rusted are getting replaced.
I spent some time today re-spacing the elements according to the new EZNEC dimensions. This didn't take long because in reality, only the two middle elements needed to move. As it turns out, the SWR ended up being exactly what the new EZNEC model predicted: The best I could get it was at about 142-143 Mhz. It would remain under 2:1 at the low end of 144Mhz, but I couldn't get it up into the middle of the band, regardless of how many different ways I adjusted the gamma match. It looks like I will have to trim all of the elements a bit.
The "Review the Gizmotchy" project has turned into a "How to Improve the Gizmotchy" project
If I get it right, the front/back ratio improvement will be worth it!I discussed in an earlier part of this review that I had some concern over the stainless steel hardware that is included. As I mentioned, it has been raining a lot here. Here is a close-up picture of what the included bolts and nuts look like now:
The U-bolts and hose clamps are all good quality stainless steel. The one bolt I had to replace is also rust-free. Before this antenna finds it's finally install location, the few bolts that have rusted are getting replaced.
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