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Messages - Lamewolf

16
I had added it to the other thread about the modified OCFD but wasn't sure anyone had seen it, so put it under a new thread to make sure.
17
Just thought I would share a couple of diagrams with the group on my modified off center fed dipole that is working great so far.  The modification is the addition of a 120pf 5KV capacitor to move the 80 meter resonance more toward the center of the band without effecting the upper bands.  The cap also has a 1 megohm 5 watt carbon resistor across it to protect it from static damage.  The diagrams also have swr readings for my particular antenna - ENJOY !
18
Finally got this thing working and here is the results in diagram form.

19
Antennas / Re: Coax Length and SWR.
April 01, 2017, 09:50:29 am
If you want a true swr reading on your antenna, then use an "ELECTRICAL HALF WAVE LENGTH" of coax which duplicates at the shack end what the impedance is at the load end.  By "electrical half wave" I mean a coax that is cut to length by factoring in the velocity factor of the given coax.  So if the velocity factor is 66%, you would do the math of 468/Freq in MHz = length in feet X .66 or 468/3.5 = 133.71 X .66 = 88.25 feet.  Once you get the antenna resonant at 3.5 MHz with that length of coax, it shouldn't matter what length of coax you use because it will be looking into a purely resistive load.  The reason different lengths of coax effect swr is because its not a purely resistive load and is effected by reactance.  Doing it this way is pretty much splitting hairs but it is more accurate !
20
Antennas / Re: Super Gain 9db 40m NVIS Antenna.
March 24, 2017, 08:46:44 am
Quote from: Quietguy on March 23, 2017, 09:11:28 pm
Quote from: DJ6KR on March 23, 2017, 10:00:54 am
It is all about the reflector on the ground under the Antenna, I think.


Yes, I have a copy of Terman's book that is referenced in the article.  Terman discusses a "Half-wave Antenna with Reflector" starting on page 903 and derives the gain.  He shows plots of gain compared to radiation resistance as a function of antenna to reflector spacing.  Radiation resistance (and efficiency) goes way down with small spacing, while gain stays around 8 up to a spacing of about 0.1 wavelength (about 13 feet on 40 m) before falling off with increased spacing.  He says:

"In order to prevent incidental loss resistances from making the antenna efficiency very low, the spacing S should accordingly be at least 0.05 wavelength, and preferably 0.1 wavelength."

The article uses 7 feet, which is about 0.05 wavelength at 40 meters, but Terman's chart shows you would have better efficiency without losing gain if you made the antenna 14 feet high.

Wally


I can see that in my minds eye, but the gain they are speaking of in this article is going straight up !  Good for NVIS, bad for DX !
21
Antennas / Re: Super Gain 9db 40m NVIS Antenna.
March 23, 2017, 08:31:45 am
I just don't see how they can claim that much gain when the antenna is basically nothing more than a half wave folded dipole.  The idea behind a folded dipole is increased bandwidth, not more gain ?
22
Antennas / Re: Random Wire Antennas Can Be Troublesome.
February 27, 2017, 04:33:32 pm
Quote from: gil on February 24, 2017, 01:20:58 pm
QuoteUse an FT140-43 core and wind 18 turns of RG174 cox on it


Good one... I have slip-on ferrites that are just the right diameter for RG-174. If that doesn't work I'll try the toroid.

Gil.


It will take a bunch of slip on beads to do the same as one toroid with those 18 turns on it.  The toroid & 18 turns would be about 6000 ohms @ 80 meters !
23
Antennas / Re: Random Wire Antennas Can Be Troublesome.
February 24, 2017, 10:15:06 am
Gil,

Use an FT140-43 core and wind 18 turns of RG174 cox on it for a very effective 1:1 choke to install between the 9:1 and the coax and run a counterpoise wire on the antenna side of the unun.
24
Antennas / Re: The Elecraft T1 Automatic Tuner.
January 11, 2017, 05:28:41 pm
Quote from: cockpitbob on December 21, 2016, 09:53:31 am
That tuner has been on my list for a while :).  /jealous

I hadn't thought about protecting the finals while it finds, or fails to find a tune.  I can't turn down the power on my MTR and I run it at 12V so it's in the danger zone for a bad mismatch.  Would you worry about using this tuner with your MTR and for example a random wire with 9:1 unun?


You guys could build a simple resistive bridge that can be switched in and out of line for tuning purposes just like the one built into Gil's ZM-2 tuner.  A few resistors and a DPDT mini toggle switch, and inline (tune position) the radio has a decent load, when done tuning, just switch it out of line to operate.  If the radio has enough room, the bridge can actually be built inside it !
25
Antennas / Re: Windom Antenna Analysis With The MR100.
January 10, 2017, 04:55:22 pm
Gil,

How do you interface the MR100 with a computer to get those nice looking graphs and what program do you use to do it with ?
26
Antennas / Re: Interesting mod for off center fed dipole
December 29, 2016, 03:12:08 pm
Quote from: madsb on December 17, 2016, 10:02:41 am
Remember that a low SWR does not indicate an efficient antenna. Otherwise I would immediately swap my dipole for a 50 ohms resistor since the resistor is both compact, lightweight , easy to set up and has a consistent low SWR across all the HF bands :)

Sendt fra min S60 med Tapatalk


True, but the resistor in  this application is not for any loading purpose since it is one million ohms, it only purpose is to protect the capacitor from static damage.
27
Antennas / Re: Interesting mod for off center fed dipole
December 17, 2016, 09:37:17 am
Quote from: gil on December 13, 2016, 06:01:50 pm
Interesting, I have never heard of this... Mine is working better now that I slipped ten ferrites over the coax, close to the middle, about 11m from the BALUN. I had to change the BNC connector so it was the perfect opportunity. Now the SWR is lower on all bands.

Gil


Thinking of experimenting with this myself.  I have a jumper with ferrite beads on it between the balun and feed line on mine, but been thinking of turning it into a Carolina Windom by moving it down the feedline enough to give me 15 meters.
28
Antennas / Re: Interesting mod for off center fed dipole
December 13, 2016, 05:57:12 pm
Quote from: gil on December 13, 2016, 09:17:19 am
Do you mean in parallel with the BALUN?

Gil


No, in series with the longer wire at the exact center of the antenna.  If the total length of the antenna is 134' then the cap and resistor goes 67' from the long end.

Rick
29
Antennas / Re: Interesting mod for off center fed dipole
December 13, 2016, 08:08:53 am
UPDATE !  When I first read about adding the capacitor, I thought it said to add it between the longer wire and the balun which did work well for 80 meters but raised the 40 meter swr.  The cap is supposed to go in the middle of the antenna, so if your antenna has a total length of 134' the cap would go 67' from the end of the longer wire.  Value of the cap can be anywhere from 100pf to 230pf depending on where you want resonance to fall on the 80 meter band - the higher the cap value, the higher the resonance is and most folks are getting good results using 200 to 220 pf.
30
Antennas / Re: Interesting mod for off center fed dipole
December 05, 2016, 03:27:47 pm
Well, I took the cap back off my windom this weekend because it actually raised the swr on 40 meters  - back to the drawing board !