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Hustler G7-144 vs G6-144 vs dipole radiation pattern
I'm curious whether anyone has run an NEC type of analysis on the Hustler
G7-144, and/or the G6-144 antennas. I'd like a gain/pattern comparison to a 1/2 wave vertically oriented dipole. However, I can't find any manufacturer pattern plots to compare these antennas. 73, de Nate -- "The optimist proclaims that we live in the best of all possible worlds, the pessimist fears this is true." |
#2
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Hustler G7-144 vs G6-144 vs dipole radiation pattern
In article ,
Nate Bargmann wrote: I'm curious whether anyone has run an NEC type of analysis on the Hustler G7-144, and/or the G6-144 antennas. I'd like a gain/pattern comparison to a 1/2 wave vertically oriented dipole. However, I can't find any manufacturer pattern plots to compare these antennas. I put together a couple of NEC models for the G7-144 a year or two ago. If I recall correctly, this antenna is composed of three 5/8-wave radiators in a collinear arrangement (stacked one above another) separated by relatively short inductive loading/phasing coils. It has quarter-wave radials at the bottom and is fed at the base through a tapped coil. I did two different models - one using shorted transmission-line stubs as the phasing elements, and another using lumped inductances. With the stub lengths, and the inductance values tweaked to give the correct current-phase relationships in the 5/8-wave radiating sections, the two models produced almost indistinguishable results in the pattern analysis, and a gain figure which matched the manufacturer's claims quite well. I can't claim that the models are precisely correct, but I'd guess that they're fairly close. The pattern seemed reasonably clean... most of the energy went straight out towards the horizon. There were minor lobes upwards and downwards at an angle of around 45 degrees, as one would expect from a 5/8-wave radiator. I didn't see any systematic "squint" (up-tilt or down-tilt) in the pattern. The "donut" of the pattern was significantly shallower than that of a half-wave reference dipole... not s surprise. I added a random wire down from the feedpoint to simulate the "RF on the outside of the coax or on the mast" issue, and found that there was little current and little effect on the pattern... according to the models, at least, the quarter-wave radials do a pretty effective job in decoupling the antenna from the mast and feedline. I compared these results against those for another antenna design I've been considering for our repeater - a pair of "plumber's delight" copper-pipe 5/4-wave extended double Zepp antennas, stacked one above the other (about a quarter-wave separating the tips of their inner arms), fed in phase. The results were roughly comparable. I've got the models on my laptop PC at home, I think... if I can find 'em I'll email them, and perhaps some results plots to you. The biggest concern I have about the G7-144 isn't its pattern - it's some aspects of its construction. The one we're using went bongo after a few years up in the weather - the repeater began suffering from extremely severe desense - it was so bad as to render the repeater unusable unless you were within a mile or so of it. We temporarily swapped the Hustler for a simple J-pole and got back most of the performance. When we tore down the G7-144 we found that the bottom end of the base-matching/feed coil was high-Z from ground. The construction of the antenna base depends on a heavy-gauge press fitting between two sections of aluminum to establish the connection between the base of the coil and the N connector, and about five years of weathering (in a fairly mild climate) had led to enough oxidation to allow the connection to go open-circuit at DC! I believe that there were also some connections which were done with ring terminals and rivets. The antenna actually worked, in the sense that it transmitted and showed a low SWR, but there was enough micro-arcing and rectification taking place in the oxidized connections to create plenty of broadband noise which was swamping the receiver and leading to many dB of desensitization. Our chief metalworking guy tore the whole thing apart, and rebuilt the base with a better N connector and a pair of soldered connections between the coil base and the ground shell of the N connector. He also cleaned up all of the sliding-tubing connections between the radiators and phasing coils, roughened them up a bit, gooped them with a conductive anti-oxidant compound, re-fit and re-tuned them, then drilled through the tubing and riveted them together, and then used heat-shrink tubing over all of the phasing-coil connections. I think he believes in wearing suspenders, a belt, *and* duct tape :-) We put it back up, and found that the desense was down to undetectable levels. It's been up in the air for 18 months now, with nary a problem. We're going to have to move our antenna tower in a few years, and at that point we'll probably retire the G7-144 and replace it with either a stacked pair of EDZ antennas (homebrew) or a stacked folded dipole system (commercial). Both of the candidate designs use fully-soldered or fully-welded radiator construction... they're DC-grounded and would be free of sliding or press-fit connections which could oxidize and degrade over time. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#3
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Hustler G7-144 vs G6-144 vs dipole radiation pattern
Thanks Dave.
That was an excellent post. It's interesting that I've read testimonials on eHam and other web pages that claim 15 to 20 years of service with the G7-144 with no complaints. Then there are some that have had some issues. The G6-144 is a bit more attractive to me right now due to the lower price and being about 2/3 of the length of the G7. Interestingly, the G6 literature (available at http://www.dxengineering.com) says, "FCC accepted for Repeater application at 6db gain based on EIA Standard RS-239: gain compared to 1/2 wave dipole." I've never heard of antennas being "FCC accepted for Repeater application" before. So now I'm curious whether the G6 may be more rugged than the G7. At the very least there won't be so much aluminum swinging in the breeze. I am looking for antenna for our repeater. We tried pressing a commercial antenna tuned for 155 MHz into service, but it just hasn't worked out as we have severe desense. Its mounting is also far from optimal. The calculated SWR at the antenna is approximately 1.8:1. We were checking things over today and everything looks good when the duplexer is terminated into a 50 ohm load, but varying levels of noise exist at the RX frequency when the antenna is connected. It is definitely our system as the spectrum is clear when the repeater TX is off. It was too windy today to climb to the top of the grain elevator leg the antenna is on to check at the end of the hardline with a wattmeter and dummy load. I like the idea of the G6 as it appears easy to handle and easy to mount. We are limited by the framework of the structure and we have to avoid having anything that would get in the way of the employees or jeopardize their safety. Also, it needs to be safe for us to work with as well. The G6 appears to be two 5/8 wave radiators fed with Hustler's fiberglass encased coil assembly. I have seen that the antenna is about 10 feet tall, weighs about 7 pounds, and has a surface area of 1.4 sq ft. We can mount a mast easily on the structure that will support a lightweight gain antenna like the G6. A DB-224 would be nice, but it requires considerably more in the way of mounting and manpower resources to do it correctly. Any suggestions are appreciated. 73, de Nate -- "The optimist proclaims that we live in the best of all possible worlds, the pessimist fears this is true." |
#4
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Hustler G7-144 vs G6-144 vs dipole radiation pattern
In article ,
Nate Bargmann wrote: Thanks Dave. That was an excellent post. It's interesting that I've read testimonials on eHam and other web pages that claim 15 to 20 years of service with the G7-144 with no complaints. Then there are some that have had some issues. The luck of the draw, I suppose? The G6-144 is a bit more attractive to me right now due to the lower price and being about 2/3 of the length of the G7. Interestingly, the G6 literature (available at http://www.dxengineering.com) says, "FCC accepted for Repeater application at 6db gain based on EIA Standard RS-239: gain compared to 1/2 wave dipole." I've never heard of antennas being "FCC accepted for Repeater application" before. I imagine that there may be some such regulations for some of the commercial or public-safety bands? Minimum or maximum gains allowed, or cleanliness-of-pattern perhaps? I am looking for antenna for our repeater. We tried pressing a commercial antenna tuned for 155 MHz into service, but it just hasn't worked out as we have severe desense. Its mounting is also far from optimal. The calculated SWR at the antenna is approximately 1.8:1. We were checking things over today and everything looks good when the duplexer is terminated into a 50 ohm load, but varying levels of noise exist at the RX frequency when the antenna is connected. It is definitely our system as the spectrum is clear when the repeater TX is off. It was too windy today to climb to the top of the grain elevator leg the antenna is on to check at the end of the hardline with a wattmeter and dummy load. It sure sounds as if you've got a less-than-good connection somewhere. We had similar problems after a while with one of our "American Legion" J-pole antennas, which use an all-aluminum construction. The radiator and matching-arm rods were originally mounted with set-screws, but on this particular antenna the connection had been reinforced with a spot-weld. Due to flexure of the rods in the breeze, the weld eventually became brittle and cracked... and the antenna immediately exhibited severe desense due to broadband-noise generation in the cracked junction. Any suggestions are appreciated. We're looking at some of Telewave's antennas, which I believe are similar to the DB-224 in their basic design approach. I'd also suggest that you take a look at the copper-pipe EDZ design at http://www.tcarc.ca/public/2mdez.php, which would have gain quite similar to the G6-144, I think. The folks who run this page seem to like this antenna for repeater service. Since it's a grounded, fully-soldered design (you can solder the balun-attachment points to the matching stub) it ought to be quite free of desense-causing corrosion/micro-arcing points. I built one of these for our repeater system, scaled down for use as a remote-linking antenna for the 440 band. Seems to have worked out well so far. The only thing I'd do differently the next time I build one, is wait to figure out the actual shorting-stub and balun attachment points until the antenna is actually in place on the tower. I tuned it in a free-space situation, and the presence of the tower de-tuned it a bit (enough to notice on a meter, not enough to matter in practice). -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#6
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Hustler G7-144 vs G6-144 vs dipole radiation pattern
On Thu, 20 Sep 2007 12:40:10 +0000, BB wrote:
There are none I know of. However, commercial systems are licensed by ERP, so I would suspect that the antenna gain figures must be accurate in order to get the ERP calculations correct. Perhaps that is what they really mean, that the gain is certified for ERP calculations. 73, de Nate -- "The optimist proclaims that we live in the best of all possible worlds, the pessimist fears this is true." |
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