| Page 1 of 2 | 1 | 2 |
|
|
5/8 wave ground plane
I want to construct a 5/8 wave ground plane for 24 MHz.
I am OK with the dimensions, but I can't get a grip on what the impedance at the feedpoint will be. Even the ARRL antenna book hasn't been much help. Can anyone tell me the impedance I can expect to see, also any tips on matching it? Thanks! Larry DiGioia N8KU N8KU at longwire.com |
For a 5/8-wave vertical above ground at 24 MHz, conductor diameter 10 mm. -
Base feedpoint resistance = 110 ohms Base feedpoint reactance = -j386 ohms Base loading coil inductance = 2.6 uH Coil diameter = 25 mm Coil length = 33 mm Number of spaced turns = 13 Wire = 14 awg = 1.64 mm diameter. For other heights, frequencies and coil dimensions download in a few seconds program BOTLOAD for immediate answers. ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
Kraus (2nd edition, p. 375) gives a different answer.
Interpolating from the diagram on p. 375 (assuming length/diameter = 472) it is about 90-j250. Why the difference? -- Jim N8EE to email directly, send to my call sign at arrl dot net "Reg Edwards" wrote in message ... For a 5/8-wave vertical above ground at 24 MHz, conductor diameter 10 mm. - Base feedpoint resistance = 110 ohms Base feedpoint reactance = -j386 ohms Base loading coil inductance = 2.6 uH Coil diameter = 25 mm Coil length = 33 mm Number of spaced turns = 13 Wire = 14 awg = 1.64 mm diameter. For other heights, frequencies and coil dimensions download in a few seconds program BOTLOAD for immediate answers. ---- .................................................. ......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. ......... |
Larry D wrote:
Can anyone tell me the impedance I can expect to see, also any tips on matching it? Thanks! The VERT1 vertical that comes with EZNEC is 1/4WL on 40m. It will be 5/8WL on 17m. (7.2 * 2.5 = 18) EZNEC sez the feedpoint impedance on 17m will be about 100-j470 ohms. A loading coil is usually installed between the base of the antenna and the radial system. The coax braid is tied to the radial system and the coax center wire is tapped down on the coil at the 50 ohm point. Rad| coil Gnd-------+-////////---------------5/8WL---------------- Rad| ^ | XMTR-----------+ If you don't mind a relatively low SWR of 2:1-3:1, use good coax, a standard base-loading coil, and your tuner at the transmitter. There is plenty of 5/8WL information for 2m antennas in the ARRL publications and antennas are scalable. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Z is close to +58-j138 ohms.
A small inductor of 0.88 uH in series with the antenna should provide a pretty good match to 50 ohm coax. Larry D wrote: I want to construct a 5/8 wave ground plane for 24 MHz. I am OK with the dimensions, but I can't get a grip on what the impedance at the feedpoint will be. Even the ARRL antenna book hasn't been much help. Can anyone tell me the impedance I can expect to see, also any tips on matching it? Thanks! Larry DiGioia N8KU N8KU at longwire.com |
Kraus (2nd edition, p. 375) gives a different answer.
Interpolating from the diagram on p. 375 (assuming length/diameter = 472) it is about 90-j250. Why the difference? ============================== The program takes into account antenna diameter. Does Kraus? And is he on the same band? Is he at the same height above ground? What sort of ground or radial system does he have? Have you interpolated correctly? Have you understood what Kraus is saying? Is the loading coil in the same location? Is the feedpoint at the same place. Are you comparing like with like? Spend more time thinking about it. Is Kraus infallible? Could be he's wrong again. Who's Kraus anyway? ;o) ---- Reg |
There is plenty of 5/8WL information for 2m antennas
in the ARRL publications and antennas are scalable. -- 73, Cecil =========================== The trouble with scaling (as from 2m down to 160m) is that antenna conductor diameters are forgotten about which can cause appreciable errors. --- Reg, G4FGQ |
Reg Edwards wrote:
The trouble with scaling (as from 2m down to 160m) is that antenna conductor diameters are forgotten about which can cause appreciable errors. I dunno, Reg. My 20m rotatable dipole diameter is about ten times bigger than my 2m 1/4WL ground plane. Seems about right to me. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Larry, N8KU wrote:
"I want to construct a 5/8 wave ground plane for 24 MHz.----Even the ARRL antenna book hasn`t been much help." At 1/2-wave, reactance nearly is zero and begins repeating as from zero.. With a 5/8-wave, you have a 1/8-wave connected to the feedpoint through a reactance repeating 1/2-wave line. A 5/8-wave antenna will have nearly the same reactance as a 1/8-wave. Value of the reactance depends on the characteristic impedance of the antenna and this depends on length to diameter ratio. Resistance of a vertical antenna increases with its length until it reaches nearly 1/2-wavelength where it peaks and starts to fall. Sharpness of this resonant peak depends on length to diameter ratio of the antenna. You might expect a radiation resistance of between 50 and 150 ohms. Reactance may be between 200 and 1200 ohms. It depends on how skinny the antenna is. Fat conductors have smaller impedance variations (a lower Q). My impedance numbers come from Capt. Paul H. Lee, USNR, K6TS`s "Vertical Antenna Handbook". 5/8-wave vertical ground-planes were once popular on 27 MHz and often used an autotransformer at the base to step up the impedance from the 50 ohms of the coax. Best regards, Richard Harrison, KB5WZI |
"Reg Edwards" wrote in message ... The program takes into account antenna diameter. Does Kraus? Yes. My post said length/diameter = 472 (One thing I don't like about the way Kraus wrote his book is that he gives the formula for only one part of the feed impedance (R or X) and a diagram for the complex impedance. He then refers you to someones paper somewhere for the 'complete' formula.. I have it around here but it would take 2 days to find it.) And is he on the same band? In the diagram I was interpolating from the dimensions are in wavelengths. Is he at the same height above ground? What sort of ground or radial system does he have? It is an infinite perfect ground plane---ground mounted. (We are talking about a 5/8 wave vertical remember). Have you interpolated correctly? Probably not. Have you understood what Kraus is saying? Yes. Is the loading coil in the same location? No 'loading coil' in the Kraus example. He is discussing the feed point impedance. A loading coil would be either across or in series with the feed point. Is the feedpoint at the same place. Since the original post concerned a 5/8 wave vertical, I assumed it is at the bottom, which is what Kraus is discussing. Are you comparing like with like? Hence my question. Are we? I recall that a 5/8 wave vertical was 'close' to 50 ohms at the feed point and could be 'adjusted' with an inductance across the feed point (with the feedline at a tap perhaps?). That is the whole point is using such a 'magic number' as 5/8 wavelength. If you look at the impedance curves, though, you find that they are quite steep at this point, so a small error in length causes a large error in impedance, and some tuning will be needed. The 5/8 wave is not a 'build it and go' antenna! Spend more time thinking about it. I have, which is why I looked it up. Is Kraus infallible? Could be he's wrong again. =8o Oh no!!! Kraus wrong!? Who's Kraus anyway? ;o) Oh, I don't know. Just some guy that signed my Master's Thesis 16 years ago. ---- Reg -- Jim N8EE to email directly, send to my call sign at arrl dot net |
Partly because you used a conductor diameter about 16.5mm and Reg's
was 10mm. (Don't think I'd want to make a self-supporting element that long and that thin, myself. Maybe 50mm at the base and tapering to 25 or so?) That probably does not account for the whole difference. Did you both use the same sort of "ground plane"? Height above earth has only a slight effect. Cheers, Tom "JLB" wrote in message ... Kraus (2nd edition, p. 375) gives a different answer. Interpolating from the diagram on p. 375 (assuming length/diameter = 472) it is about 90-j250. Why the difference? -- Jim N8EE to email directly, send to my call sign at arrl dot net "Reg Edwards" wrote in message ... For a 5/8-wave vertical above ground at 24 MHz, conductor diameter 10 mm. - Base feedpoint resistance = 110 ohms Base feedpoint reactance = -j386 ohms Base loading coil inductance = 2.6 uH Coil diameter = 25 mm Coil length = 33 mm Number of spaced turns = 13 Wire = 14 awg = 1.64 mm diameter. For other heights, frequencies and coil dimensions download in a few seconds program BOTLOAD for immediate answers. ---- .................................................. ......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. ......... |
The VERT1 vertical that comes with EZNEC is 1/4WL on 40m.
It will be 5/8WL on 17m. (7.2 * 2.5 = 18) EZNEC sez the feedpoint impedance on 17m will be about 100-j470 ohms. Hi Cecil, He was asking about 24MHZ which is 12 meters. EZNEC sez a 5/8 on 24MHZ is 63-j206, more or less. As others have pointed out a lot of factors will enter into the final installed impedance. Another matching method, for 63-j206, is feed the antenna with a 1.2 meter length of 300 ohm twinlead- 1:1 balun- 50 ohm coax. The twinlead transformer gives 43-j0. Don't tell Reg, but I did that on the Smith Chart in 48.32 seconds:-). 73 Gary N4AST |
JLB wrote:
Hence my question. Are we? I recall that a 5/8 wave vertical was 'close' to 50 ohms at the feed point and could be 'adjusted' with an inductance across the feed point (with the feedline at a tap perhaps?). Tuned to an electrical 3/4 wave...Basically, just tuning out the reactance.. That is the whole point is using such a 'magic number' as 5/8 wavelength. Huh? The point of the 5/8 radiator is pattern and gain related, not feed impedance, or matching. If you look at the impedance curves, though, you find that they are quite steep at this point, so a small error in length causes a large error in impedance, and some tuning will be needed. The 5/8 wave is not a 'build it and go' antenna! It's pretty close to it though. All you have to do is start off with a "best guess" coil and test. If it's resonant low in freq, take turns off. One at a time, if close. If it's high in freq, you need more turns. Not really any more complicated than trimming a dipole for resonance. After building a lot of those, I can guess the number of turns needed in my head and come pretty close. Reg's guess of 13 turns is pretty close if the form is 1-1.5 inch wide. Using my built in guessing machine, I would have quoted appx 15-16-17 turns off the top of my head , if the coil is made of #12 solid wire, and wound on a 7/8 inch pipe former, "I use a piece of 7/8 inch copper tube as the former" and then taken off the form. "On those type of coils, I make them self supporting. The #12 wire is stiff enough to hold it's shape. Just connect and clamp each end of the coil to hold it in place. MK -- http://web.wt.net/~nm5k |
JGBOYLES wrote:
Hi Cecil, He was asking about 24MHZ which is 12 meters. EZNEC sez a 5/8 on 24MHZ is 63-j206, more or less. I had forgotten I had changed VERT1 from tubing to #14 wire. That was the problem with the higher impedance. With 3 inch dia. vertical tubing the impedance on 18 MHz is 62-j190. -- Sorry & 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Several posters have recommended a simple series coil for matching.
But of course, you'll never get lower than the resistive component of the feedpoint impedance that way. At least one suggested a tapped coil, and that can give you a good match and has the advantage of DC-grounding the lightening ro...I mean radiating element. Another alternative is a shunt coil to ground and a series capacitor off to the feedline; that gives you the grounding and also lowers the low-frequency coupling to the feedline. If Reg's suggested 110-j386 is correct for your case, you can get a match to 50 ohms at 24MHz with a 1.1 microhenry shunt coil and 25pF series capacitor. The Q should be around 5, not much higher than the antenna itself but can lead to fairly high voltage across the cap. Don't leave it exposed to rain :-) (That's an advantage of the tapped coil: less suseptable to weather problems.) But beware...the range of impedances I've seen posted suggests your installation may be somewhat different. Cheers, Tom Larry D wrote in message ... I want to construct a 5/8 wave ground plane for 24 MHz. I am OK with the dimensions, but I can't get a grip on what the impedance at the feedpoint will be. Even the ARRL antenna book hasn't been much help. Can anyone tell me the impedance I can expect to see, also any tips on matching it? Thanks! Larry DiGioia N8KU N8KU at longwire.com |
On 12-May-2004, "JLB" wrote: Hence my question. Are we? I recall that a 5/8 wave vertical was 'close' to 50 ohms at the feed point and could be 'adjusted' with an inductance across the feed point (with the feedline at a tap perhaps?). That is the whole point is using such a 'magic number' as 5/8 wavelength. The resistive part of the Z of a 5/8 wave will closer to 50 ohms than it will be for a 1/2 wave, but in most cases will be higher than 50. You may be thinking of a vertical close to 3/8 wave which will be 50 -jx. The -jx (capacitance) can be resonated with a +jx in series for a 50 ohm feedpoint. Ken Fowler |
You may be thinking of a vertical close to 3/8 wave which
will be 50 -jx. The -jx (capacitance) can be resonated with a +jx in series for a 50 ohm feedpoint. Ken, we may be talking about two different things, but a 3/8 wave vertical has inductive reactance, and the R component is no where near 50 ohms. A 2/8, or 1/4 wave vertical has no reactance, shorter has -jx, and longer, as in 3/8 wave has +jx. A .26 lambda vertical has about 50+j43 impedance, and the +j43 can be cancelled with series 154pf at 24 MHZ. This is a common method of matching a 1/4 wave vertical to 50 ohms. Make it a little longer to look inductive, 50+jx, and cancel with a series cap. According to all the theory I know on this stuff a 3/8 wave vertical will not be as you describe. 73 Gary N4AST |
|
Cecil Moore wrote in message ...
Larry D wrote: Can anyone tell me the impedance I can expect to see, also any tips on matching it? Thanks! The VERT1 vertical that comes with EZNEC is 1/4WL on 40m. It will be 5/8WL on 17m. (7.2 * 2.5 = 18) EZNEC sez the feedpoint impedance on 17m will be about 100-j470 ohms. This is what I ran when I had my 40m GP up. It worked both 17m and 40m. My old elnec model of that antenna showed appx 48.4-j213 ohms on 17m. But I had sloping radials. Maybe that made a difference. I used an ungrounded series coil to tune for 17m, and the SWR was very low. I had a 24v DPDT relay that I used to bypass the coil for 40m use. Otherwise, it would be resonant about 6.9 mhz. I had the transformer here in the shack, and when I changed bands, I would just plug, or unplug the transformer from my power strip. MK |
|
|
I'm not sure about your 50 + j43 for a 0.26 wave. Is that from EZNEC? With
what kind of ground? Yes it was EZNEC, ground type real/Mininec. 73 Gary N4AST |
"Larry D" wrote in message ... I want to construct a 5/8 wave ground plane for 24 MHz. I am OK with the dimensions, but I can't get a grip on what the impedance at the feedpoint will be. Even the ARRL antenna book hasn't been much help. Can anyone tell me the impedance I can expect to see, also any tips on matching it? Thanks! Larry DiGioia N8KU N8KU at longwire.com Larry, its been a while since I built one but I usually wind an 1/8 to 3/16wl of wire on my matching coil and feed this in series with as one would a base loaded antenna then start triming the coil for lowest SWR. I have one I built like that now on 10M SWR is about 1.5:1 . Cant say for sure but think the feed point impedance is above 50 or about 75 ohms at resonance. Have built these using the same technique for 6 and 2 . Seems like the impedance on these can vary quite a bit depending on the materials you build the antenna from. Thicker materials seems to give a lower impedance and a better match to 50 ohms. Ive never really gone into what was going on with the antenna that much and may be all wrong but right or wrong my way of doing it seems to get the job done for me. Perhaps some of the gurus around here would like to provide some more insight. |
Dave Shrader wrote:
Z is close to +58-j138 ohms. A small inductor of 0.88 uH in series with the antenna should provide a pretty good match to 50 ohm coax. Larry D wrote: I want to construct a 5/8 wave ground plane for 24 MHz. Wow, after all that, a simple answer. Thanks! The reason I wanted to try 5/8 wave is because I have had the Solarcon A99 1/2-wave ground plane for 10 meters up for about 5 years, and it is an excellent antenna for DX. This is, according to the product blurb, a "1/2 wave, voltage-fed" design. The radiation patterns in the antenna book give the impression that 5/8 wave should be "even better" for DX. Of course, a lot of that performance had to do with who is on 10 meters in the first place, and conditions over the past 5 years as well. It is finally falling apart (radials coming loose,) and the prospects for 10m in the next few years don't look so good. I wanted to switch to a similar antenna, but for 24 MHz. I notice that some of the answers here were geared toward a ground-mount, which is not what I meant by "ground plane," this antenna will be at 30', and probably will have full-size, 1/4 wave (?) sloping radials. As it turns out, at the antenna forum in Dayton, Dean Straw, the author of the ARRL antenna book, talked me out of using 5/8 wave. He feels that it will not appreciably enhance DX, compared to 1/4 wave. He also talked me into getting the latest version of the antenna book, which alas, still does not mention doing what you suggest, not even in the VHF section, as one person mentioned, but at least I have the answer now. Thanks again. Larry DiGioia N8KU |
Larry D. wrote:
"The reason I wanted to try 5/8 wave is because I have had the Solarcon A 99 1/2 wave ground plane for 10 meters up for about 5 years and it is an excellent antenna for DX." OK. A comparison of unattenuated field strengths at one mile from various verticals is given on page 20 of the "Vertical Antenna Handbook" by Capt. Paul H. Lee, USNR, K6TS: 1/4-wave, 196 mv/m 1/2-wave, 236 mv/m 5/8-wave, 276 mv/m Power varies as the square of the voltage. One can see the difference in the field strength is hardly worth the effort for an amateur to try to increase the length of his antenna. It`s about a 3 dB gain from 1/4-wave to 5/8 wave. A 5/8-wave at 10 meters is 1/2-wave at 12.5-meters (24 MHz). A 1/2-wave at 10-meters is 0.4 wavelength at 12.5-meters (24 MHz). One could resonate it with a series capacitance at the antenna. Best regards, Richard Harrison, KB5WZI |
|
Dan Richardson wrote in message . ..
On Mon, 17 May 2004 10:42:49 -0500, (Richard Harrison) wrote: [snip] Power varies as the square of the voltage. One can see the difference in the field strength is hardly worth the effort for an amateur to try to increase the length of his antenna. It`s about a 3 dB gain from 1/4-wave to 5/8 wave. [snip] The 3 dB gain figure is valid when mounted on theoretical perfect ground. For a ground-plane elevated above real ground you'll find the gain to be rarely greater than 1 dB. Dunno. My real world tests don't quite agree. In using 30 mile ground wave tests across town, I tested 1/4 GP's, 1/2 waves including decoupling sections, and a 5/8 GP with 3/4 wave radials. All at 36 ft. The 5/8 ate the 1/4 GP for lunch. Probably 2 plus S units better than the 1/4 GP. The 5/8 beat the 1/2 wave by 1.5 S units. And this was tested and repeated over a period of months. Never varied. Ground wave testing is quite stable, and accurate for those low angles involved. Much more accurate than trying to compare using constantly varying skywaves. In real world gains, thats more than 1 db. 5/8 antennas are weird animals. On 2m, they suck. On HF, they can do fairly well, cuz the angles involved are not as critical. I used a 5/8 GP on 17m for 2-3 years. "also at 36 ft at the base". It mangled every other antenna I had on that band. On 10m, the 5/8 beat any other length radiator quite handily. Again, on the critical 2m band, peeeyooooo.....they stink. BTW, on skywave, using a quick A/B test, all preferred the 5/8, over the other antennas. So it wasn't a low angle ground wave fluke. MK |
Dan Richardson wrote in message . ..
On Mon, 17 May 2004 10:42:49 -0500, (Richard Harrison) wrote: [snip] Power varies as the square of the voltage. One can see the difference in the field strength is hardly worth the effort for an amateur to try to increase the length of his antenna. It`s about a 3 dB gain from 1/4-wave to 5/8 wave. [snip] The 3 dB gain figure is valid when mounted on theoretical perfect ground. For a ground-plane elevated above real ground you'll find the gain to be rarely greater than 1 dB. Modeling 1/4 & 5/8-wave ground planes at 30 feet above ground (@ 24 MHz) as was described earlier NEC reports the following maximum gain: 1/4-wave 2.35 dB 5/8-wave 3.06 dB Then howcum my 5/8 wave mag mount 2m mobile antenna very significantly outperforms the 1/4 wave mag mount antenna I used to use? Same ground condx, same power, same feeder length, same vehicle, etc. I suspect it's in the differences in the TO angles. 73 Danny, K6MHE Brian w3rv |
|
I agree completely. Question is, if modeling and prediction is so unreliable
why do we bother? "Dan Richardson @mendolink.com" ChangeThisToCallSign wrote in message ... On 18 May 2004 06:19:50 -0700, (Brian Kelly) wrote: Then howcum my 5/8 wave mag mount 2m mobile antenna very significantly outperforms the 1/4 wave mag mount antenna I used to use? .... My previous response was for a ground plan antenna mounted above ground and you are addressing a mobile installation. They are different. ... I found that the vehicle's size, shape and whip location plays a major part in performance. .... it was possible to find azimuth directions that a 5/8-wave would produced almost 3 db gain over itself .... I don't feel you can accurately predict how a the whips will perform on a vehicle based upon operation on another vehicle.... |
Well -- I have found that modelling gets you in the ball park (an
approximation) A good SWR meter will allow you to tweak it up. For 1/4 wave vs 5/8 wave 2M antennas -- I have found empirically -- that when in an area surriunded by big mountain tops -- like Silicon Valley in the CA Bay area -- the 1/4 wave works better due to its higher angle of radiation of the 1/4 wave When on the open road where terrain is flat and the mountains are far away -- the 5/8 wave works best -- lower angle of radiation. This is just my empirical observation --- your radiation angle may vary -- hi hi. -- Caveat Lector Ya All "Amateur Radio is the best back-up communications system in the world, and that's the way it is." -- Walter Cronkite "Vito" wrote in message ... I agree completely. Question is, if modeling and prediction is so unreliable why do we bother? "Dan Richardson @mendolink.com" ChangeThisToCallSign wrote in message ... On 18 May 2004 06:19:50 -0700, (Brian Kelly) wrote: Then howcum my 5/8 wave mag mount 2m mobile antenna very significantly outperforms the 1/4 wave mag mount antenna I used to use? .... My previous response was for a ground plan antenna mounted above ground and you are addressing a mobile installation. They are different. ... I found that the vehicle's size, shape and whip location plays a major part in performance. .... it was possible to find azimuth directions that a 5/8-wave would produced almost 3 db gain over itself .... I don't feel you can accurately predict how a the whips will perform on a vehicle based upon operation on another vehicle.... |
On Tue, 18 May 2004 12:29:46 -0400, "Vito" wrote:
I agree completely. Question is, if modeling and prediction is so unreliable why do we bother? Hi OM, There is modeling, and then there are modelers. 99.9% of errors are found with the second. A simple example that explains the illusion of disparity may be tested with the free version of EZNEC (as is generally the case). Model a ground plane antenna at ground level (or simply an inch or cm above it); and then raise the same antenna a quarter wave (not so difficult to manage at these breathless reports from VHF-land). Difference approaches 6dB for this trivial exercise alone. Models answer the differences quite well, modelers can be found in commercials wearing lab coats saying "I'm not a doctor, but...." 73's Richard Clark, KB7QHC |
On Tue, 18 May 2004 12:29:46 -0400, "Vito" wrote:
Question is, if modeling and prediction is so unreliable why do we bother? Why do you feel modeling is "so unreliable"? Danny, K6MHE |
Dan Richardson wrote in message . ..
On 18 May 2004 06:19:50 -0700, (Brian Kelly) wrote: Then howcum my 5/8 wave mag mount 2m mobile antenna very significantly outperforms the 1/4 wave mag mount antenna I used to use? Same ground condx, same power, same feeder length, same vehicle, etc. I suspect it's in the differences in the TO angles. My previous response was for a ground plane antenna mounted above ground and you are addressing a mobile installation. They are different. Agreed. I've done modeling of 1/4,1/2 and 5/8-wave whips using several wire grid models for vehicles (small and mid-sized car, small pickup truck and a SUV). I found that the vehicle's size, shape and whip location plays a major part in performance. No surprise there although my instincts tell me that once some number of "critical" square feet of vehicle sheet metal is "achieved" the size of the vehicle has a less pronounced effect particulary at VHF and UHF freqs. But modeling has the annoying ability to deflate instincts. I also noted, on the average, the 5/8 produce a slight gain over all, but it was possible to find azimuth directions that a 5/8-wave would produced almost 3 db gain over itself - depending what vehicle it was mounted on. That would easily explain my offhand experiences. In fact I did it again last weekend. An buddy of mine is temporarily laid up in a skilled care facility so I got his home 2M FM station running in his room. I installed his 1/4 wave magmount whip on one of those typical steel-shrouded HVAC units often found under the windows of patient rooms. He couldn't hit the repeater. I went back later and installed my 5/8 wave whip and yup, now he can hit the repeater. The most outstanding feature I saw was a 5/8-wave whip azimuth pattern was less influenced by the vehicle geometry. The operation is more like a lop-sided dipole with the vehicle body being on leg. I don't feel you can accurately predict how a the whips will perform on a vehicle based upon operation on another vehicle - unless both vehicles and the antenna locations are the same. I should add for all models the whips where placed top-dead-center of the vehicle's roof. 100% agreed and it all fits. Tnx. 73, Danny, K6MHE Brian w3rv |
Why do you feel modeling is "so unreliable"?
========================= Because 99.99 percent of models are never properly tested in practice and in an anateur's back yard, if they WERE tested, the radiation patterns would only bear a vague resemblance to predictions. Predictions are mostly wishful thinking. Nevertheless, they can be a satisfying intellectual activity. ---- Reg. |
On Tue, 18 May 2004 20:05:37 +0000 (UTC), "Reg Edwards"
wrote: Why do you feel modeling is "so unreliable"? ========================= Because 99.99 percent of models are never properly tested in practice and in an anateur's back yard, if they WERE tested, the radiation patterns would only bear a vague resemblance to predictions. Predictions are mostly wishful thinking. Nevertheless, they can be a satisfying intellectual activity. ---- Reg. I see........ Then I assume then that your diagnosis of computer modeling applies to your antenna computer programs as well? Danny |
"Dan Richardson wrote Why do you feel modeling is "so unreliable"? ========================= Because 99.99 percent of models are never properly tested in practice and in an anateur's back yard, if they WERE tested, the radiation patterns would only bear a vague resemblance to predictions. Predictions are mostly wishful thinking. Nevertheless, they can be a satisfying intellectual activity. ---- Reg. I see........ Then I assume then that your diagnosis of computer modeling applies to your antenna computer programs as well? Danny ==================================== Danny, there's nothing wrong with antenna pattern-prediction progams. There are good nunber-crunched side effects. It's only their practical application by people in their own back yards which is unreliable. It cannot be helped. It's a fact of life. The environment and performance of an antenna cannot be accurately predicted unless it is at a height of several wavelengths above and away from obstructions. Not a single one of my programs predicts a radiation pattern. Only an exceedingly few professional antenna design engineers would ever make real use of such facilities even if I could write them. I don't like wasting time. I havn't all that amount of time anyway. Nobody has produced a new type of antenna for many decades. (Not even Fractal). It's all been done before. But, as I say, radiation prediction programs are entertaining, educational and satisfying. Enjoy them while you can. Nevertheless, unreliable is a fair practical description. ---- Reg, G4FGQ |
On Tue, 18 May 2004 23:50:17 +0000 (UTC), "Reg Edwards"
wrote: But, as I say, radiation prediction programs are entertaining, educational and satisfying. Enjoy them while you can. If you will review my posting of the results i reported you will see that I was comparing 1/4, 1/2 and 5/8-wave whips under the same conditions for each antenna. I was not and did not report the reported gain figures, rather, I posted the difference or lack there of for the comparisons. Under those conditions I feel comfortable stating that difference between using a 1/4, 1/2 or 5/8-wave whips on a vehicle (mounted top-dead-center on the roof) the overall gain differences would be one dB or less. I have observed, operating mobile using the above antennas that I really couldn't notice any real difference. Bad areas remained bad areas and I would loose the repeater in about the same location regardless which antenna I used. In other words I was comparing apples to apples and not looking for an absolute gain value, rather, the difference and I am very comfortable with the findings - both via computer modeling and actual use. very 73 Danny, K6MHE |
You are only sort of correct.
At HF where the local world is as large as your antenna, that statement is true, at 6m it starts to become less of an issue as the outside world becomes big and somewhat ignorable. At 2m it is quite ignorable, and at 70cm it pretty much isn't there. Yagis I have designed for 432 perform exactly as predicted by the software, with the exception of the ones too long for the antenna range, where the only discrepency was the gain tests a bit lower than the model predicts. And then it was .3 dB on an 18.4 dBd prediction. It is possible to predict antenna gain and pattern very accurately if you do it at a frequency that does not interact with the local environment to a any large degree. It is also possible to build those antennas and have them perform the same time and time again. I've built dozens of long boom yagis for 2, 222, and 432, and after the first one, you don't even have to measure the SWR before setting the T match shorts unless you are anal or an EME're. Oops, same thing, I guess. ;) And every antenna I've ever range tested (thank you Central States VHF) has been within .1 dB of predicted except one. The designs I have built over the last 15 years didn't need to be adjusted at all from the model plus predicted boom corrections. The only variable was the matching system on the driven element, which none of the modeling programs handles very well. And for you picky folks, yes, the losses from foliage, etc. aren't ignorable, but the antenna system still performs as predicted unless you mount it in the center of a tree. And I do realize I'm preaching to the non-choir here, since this group is (apparently) not very interested in anything above 10m. :) tom K0TAR Reg Edwards wrote: ==================================== Danny, there's nothing wrong with antenna pattern-prediction progams. There are good nunber-crunched side effects. It's only their practical application by people in their own back yards which is unreliable. It cannot be helped. It's a fact of life. The environment and performance of an antenna cannot be accurately predicted unless it is at a height of several wavelengths above and away from obstructions. Not a single one of my programs predicts a radiation pattern. Only an exceedingly few professional antenna design engineers would ever make real use of such facilities even if I could write them. I don't like wasting time. I havn't all that amount of time anyway. Nobody has produced a new type of antenna for many decades. (Not even Fractal). It's all been done before. But, as I say, radiation prediction programs are entertaining, educational and satisfying. Enjoy them while you can. Nevertheless, unreliable is a fair practical description. ---- Reg, G4FGQ |
Oh, I forgot the important bit - I did it in my back yard.
tom K0TAR Tom Ring wrote: You are only sort of correct. At HF where the local world is as large as your antenna, that statement is true, at 6m it starts to become less of an issue as the outside world becomes big and somewhat ignorable. At 2m it is quite ignorable, and at 70cm it pretty much isn't there. Yagis I have designed for 432 perform exactly as predicted by the software, with the exception of the ones too long for the antenna range, where the only discrepency was the gain tests a bit lower than the model predicts. And then it was .3 dB on an 18.4 dBd prediction. It is possible to predict antenna gain and pattern very accurately if you do it at a frequency that does not interact with the local environment to a any large degree. It is also possible to build those antennas and have them perform the same time and time again. I've built dozens of long boom yagis for 2, 222, and 432, and after the first one, you don't even have to measure the SWR before setting the T match shorts unless you are anal or an EME're. Oops, same thing, I guess. ;) And every antenna I've ever range tested (thank you Central States VHF) has been within .1 dB of predicted except one. The designs I have built over the last 15 years didn't need to be adjusted at all from the model plus predicted boom corrections. The only variable was the matching system on the driven element, which none of the modeling programs handles very well. And for you picky folks, yes, the losses from foliage, etc. aren't ignorable, but the antenna system still performs as predicted unless you mount it in the center of a tree. And I do realize I'm preaching to the non-choir here, since this group is (apparently) not very interested in anything above 10m. :) tom K0TAR Reg Edwards wrote: ==================================== Danny, there's nothing wrong with antenna pattern-prediction progams. There are good nunber-crunched side effects. It's only their practical application by people in their own back yards which is unreliable. It cannot be helped. It's a fact of life. The environment and performance of an antenna cannot be accurately predicted unless it is at a height of several wavelengths above and away from obstructions. Not a single one of my programs predicts a radiation pattern. Only an exceedingly few professional antenna design engineers would ever make real use of such facilities even if I could write them. I don't like wasting time. I havn't all that amount of time anyway. Nobody has produced a new type of antenna for many decades. (Not even Fractal). It's all been done before. But, as I say, radiation prediction programs are entertaining, educational and satisfying. Enjoy them while you can. Nevertheless, unreliable is a fair practical description. ---- Reg, G4FGQ |
| All times are GMT +1. The time now is 10:37 PM. |
|
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
RadioBanter.com