Never use a spring. They always have garbage wire that you do not see inside the spring that carries the signal. Water always get's into them and the wire always breaks from movement. If you need to add inches to the length for resonance just use a sold chunk of flat or bare stock. No magic in a cheap rusty spring or a cheaply made ball mount. If you want to see erratic behavior I dare someone to hook up that set up to spectrum analyzer and watch it while driving down the road keying it! The resistance will vary all over the place with deviation from vertical the more it varies the greater the deviation which changes how the tank circuit behaves. You could also start with 108 inch whip. Most people that use 102" whips for ground planes will use wire radials of 108 inches.
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Help with ground plane for 102 inch whip
- Thread starter Jlwhitt77
- Start date
I had dual co-phased S.S. whips on a 1965 Dodge Carry all. One of the springs developed and intermittent SWR problem. I took some Braided ground straps and attached S.S. ring lugs on both ends and jumped the spring with them. The SWR problem was gone after that. Easier and cheaper than replacing the springs. I needed those springs because if I went under a bridge that was less the 12'6" my whips would hit the bridge. Same as this truck but painted school bus yellow and the ball mount was above the contour of the rear wheel well.
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Please do not mistake VSWR with Resonance.
The feed point of a 1/2 wave dipole 1/2 wave agl should be around 72 ohms and decrease as lowered. However it is resonant @ 72 ohms . At higher elevations and larger spacing between the feed point all kinds of variables come into play.
The first, best way to verify this is a sweep with an analyzer. The next best way is to use the substitution method. IE: have a selection of resistors on hand so that you can make up a resistive load that approximates your VSWR readings at maximum received signal. Just go looking for the most noise on a vacant frequency. The problem with the substitution method is you have to guess between one of two answers . A VSWR meter will show 3:1 at 150 ohms or at 16.6 ohms so a bit of intuition is required.
Okay so none of this helps much with something in the 1.5:1 to 3:1 indication so on to options.
1:1 current balun at feed point.
This option is universally because the dipole is a balanced antenna and the coax is an unbalanced line.
The stub match at the feed point. Once the feed point impedance is established then comes the math for a open or closed stub to transform the impedance to the feed line.
Transmission line transformer . Usually a 1/4 wave section of 75 ohm coax (remember to do the math for .66 or .79 velocity factor) beginning at the feed point and connecting to the feed line.
An antenna tuner at the generator. Just accept the VSWR loss on the line and transform the impedance at the shack. The radio/amp is happy but you are accepting whatever losses on the coax. I recommend a Pi-L type over a "T" type tuner for some harmonic suppression as well as matching.
Of all of these the transmission line transformer is easiest, The stub match keeps the VSWR off the line but is a bit narrow banded, The balun is the most broad banded but usually requires tuner in the shack .
Then again I'm possibly full of it ...
The feed point of a 1/2 wave dipole 1/2 wave agl should be around 72 ohms and decrease as lowered. However it is resonant @ 72 ohms . At higher elevations and larger spacing between the feed point all kinds of variables come into play.
The first, best way to verify this is a sweep with an analyzer. The next best way is to use the substitution method. IE: have a selection of resistors on hand so that you can make up a resistive load that approximates your VSWR readings at maximum received signal. Just go looking for the most noise on a vacant frequency. The problem with the substitution method is you have to guess between one of two answers . A VSWR meter will show 3:1 at 150 ohms or at 16.6 ohms so a bit of intuition is required.
Okay so none of this helps much with something in the 1.5:1 to 3:1 indication so on to options.
1:1 current balun at feed point.
This option is universally because the dipole is a balanced antenna and the coax is an unbalanced line.
The stub match at the feed point. Once the feed point impedance is established then comes the math for a open or closed stub to transform the impedance to the feed line.
Transmission line transformer . Usually a 1/4 wave section of 75 ohm coax (remember to do the math for .66 or .79 velocity factor) beginning at the feed point and connecting to the feed line.
An antenna tuner at the generator. Just accept the VSWR loss on the line and transform the impedance at the shack. The radio/amp is happy but you are accepting whatever losses on the coax. I recommend a Pi-L type over a "T" type tuner for some harmonic suppression as well as matching.
Of all of these the transmission line transformer is easiest, The stub match keeps the VSWR off the line but is a bit narrow banded, The balun is the most broad banded but usually requires tuner in the shack .
Then again I'm possibly full of it ...
The way I measured resonance on that system was by working up a radiation pattern with a RF voltmeter. Take a reading at a distance make a change take another reading and compare. If the field strength increases significantly change it just a little more and compare again. If computers had been available might have been able to calculate efficiencies. I went for the peak of field strength not a dip in SWR. I kept the SWR low to protect the finals in my radio.
What I was discussing above is when I was driving down the road the SWR would fluctuate in a manner that was not characteristic of this system. The ground strap with the ring lugs simply salvaged the springs. Money was tight back then, but I never did replace those jumped springs. If it ain't broke no more don't fix it, Yah?
What I was discussing above is when I was driving down the road the SWR would fluctuate in a manner that was not characteristic of this system. The ground strap with the ring lugs simply salvaged the springs. Money was tight back then, but I never did replace those jumped springs. If it ain't broke no more don't fix it, Yah?
I also use a field strength meter to dial in peak gain on pretty much every antenna I tune. If you want the antenna working at a level of maximum transmitted power, this really is the only way to do it.
A lot of people think that the low SWR point has the most gain, others, who have a little more knowledge, will tell you it is the resonant point that has the most gain. Both of these groups are in fact wrong, the point with the most gain doesn't necessarily line up with either of these points, although it can.
All this being said, on a vast majority of antenna installations, you won't notice the difference between tuning between any of these three points.
The DB
A lot of people think that the low SWR point has the most gain, others, who have a little more knowledge, will tell you it is the resonant point that has the most gain. Both of these groups are in fact wrong, the point with the most gain doesn't necessarily line up with either of these points, although it can.
All this being said, on a vast majority of antenna installations, you won't notice the difference between tuning between any of these three points.
The DB
Thanks for the corrections I was quite intoxicated when I typed my response. I mean what I said though about avoiding springs and that 108 inch radials at 45 degree's if using for a base station ground plane works well. Also my experience with Springs no matter the antenna has always been negative! Durability is also a huge consideration for me.Springs are even worse for fiberglass rod antenna because the spring loads up nicely for a second impact usually breaking them long before swr issues can develop! I suspect that back in the 1960's and 1970's high-quality ball mounts where widely available but the stuff I have seen post 1990's has been less than high quality.