For a 40m dipole model (length 66 ft, diameter 1 inch, aluminum, 29 segments, over "Average" ground) the chart below shows (red trace) the frequency of minimum SWR(50) and (blue trace) the frequency
Author: TexasRF--- via TowerTalk <towertalk@contesting.com>
Date: Mon, 30 May 2016 13:29:46 -0400
Jim, that was 30 inches to 60 feet. Gerald K5GW In a message dated 5/30/2016 11:54:48 A.M. Central Daylight Time, jim.thom@telus.net writes: ground. [snip] Finally on 40 meters I saw as much as a 300
Jim, With my 2EL Force 12 40M yagi, I followed Tom Schiller's guidelines and set it up initially at 15 feet above the ground. When I raised it up to 70 feet the resonance frequency increased by 20khz
Jim, that was 30 inches to 60 feet. Gerald K5GW I was using two distinct points on his chart, 30 FEET.....and 100 FEET. Resonant freq at 30 ft = 7030 Resonant freq at 60 ft = 7240 http://s33.postimg.
Author: Dan Maguire via TowerTalk <towertalk@contesting.com>
Date: Tue, 31 May 2016 08:27:27 +0000 (UTC)
I sure don't see anything in the model that would tax the capabilities of NEC so I guess we'll have to agree to disagree on that. However, in playing around a bit more I did stumble on something that
The purpose of the saw horses was to assemble the antennas - not to be used for any real testing beyond that other than a sanity check that things aren't way off (i.e. 10:1 SWR). I do find that testi
now this is where I get a bit frustrated ... MY experience in modeling height change is that it doesn't reliably predict it! I am using NEC2 (although models checked have been re-checked courtesy of
AH HA! Ok - so there is a difference between a dipole and a yagi. Great to know! And if indeed that new model re a yagi shows similar results that would be quite encouraging. My apologies if I missed
I sure don't see anything in the model that would tax the capabilities of NEC so I guess we'll have to agree to disagree on that. However, in playing around a bit more I did stumble on something that
Perhaps this debate is really a measurement problem -- I think the important thing to remember is that a SWR bridge is probably NOT going to measure resonance when attached to some random length of c
rare cases, a 1-2 ft jumper. Done on each ele of the 40m yagi. But 40m els removed from boom, and hauled up the tower..one at a time. 80m rotary dipole mounted almost to top of mast..same deal thoug
Force12 sure has complicated way to assemble a standard 40m shorty beam! I agree that is sounds strange that a dipole would shift 200+Khz moving it from 30' to 60' however my quick EZNEC+ model shows
Exactly right, Grant. I regularly measure Z in the shack with my DG8SAQ-designed VNWA, follow that with a TDR measurement of the line at VHF, then use Dan's ZPlots with short and open measurements of
Part of the problem is that some people consider "resonance" and "matched to the transmission line" (i.e. 1:1 VSWR) as being one in the same when they are talking about antennas, when they most deci
A little sloppy, I should have said "transforms to some other SWR on an SWR bridge". We agree there are a couple of much better measurement means. Your "lacking that" example still requires a measuri
Author: Dan Maguire via TowerTalk <towertalk@contesting.com>
Date: Wed, 1 Jun 2016 08:30:05 +0000 (UTC)
Well, now we're making some progress at reaching common ground. I created a dipole model with 1.5" diameter aluminum wires and adjusted the length for precise resonance at 7.15 MHz in free space. The
Dan, If you have the time, you might want to try plotting resonant frequency vs height for a linear half-wave dipole and comparing it with one bent into an almost closed square, as in the "cobweb" de
Interesting. I trust antenna modeling but I wonder why linear loading is less influenced by the ground. I also wonder if coil loaded elements react the same. John KK9A 100% length (69.79 ft), no load
Just found one a plot I did a while ago: http://www.karinya.net/g3txq/temp/dipole_vs_cobweb_tuning.png Steve G3TXQ If you have the time, you might want to try plotting resonant frequency vs height fo
Force12 sure has complicated way to assemble a standard 40m shorty beam! I agree that is sounds strange that a dipole would shift 200+Khz moving it from 30' to 60' however my quick EZNEC+ model shows