Sometimes... it's the simple things that you just can't do...

[guitar_199]

Backing story: About 10 years ago at a garage sale at the end of my block... I bought a Wilson 1000 Mag Mount for $10. I have tried to set it before with just an SWR meter and it always seemed tricky... like it didn't want to behave. When I first got it... the coax was pretty ragged. So I dismantled the thing, removed the RG/58 and soldered in a nice length of RG/8X. No shorts in my coax job (long pants only!!!). Buttoned it all up.

Fast forward to tonight.....

I am out there with this W 1000 and an MFJ antenna analyzer. No matter WHAT I DO.... the center is up in the upper 28 Mhz to 29 Mhz area...
So.... the whip is too short .... right? Well this one is bottomed out!!!!!!!! And still it is tuning WAY up there....

So I wonder if somebody sawed off the whip???

Visually.. it doesn't look like it. So I look up the specs. Every ad that I see says that the whip should be 62". So excuuuuuuuuuse me while I whip out my tape measure. MINE measures about 65"..... WHAT???? Did the previous owner swap the right one out with a longer one????

But....wait...... this things is resonant in the 29 Mhz area..... a long whip WOULD NOT add up to an antenna resonating too high!!!!!

So I am sitting here thinking..........about ALL the WARS I have seen over COAX LENGTH.........

Would my replacing the original RG/58 coax with a random length of RG/8X be causing this thing to tune high like that???? To ME it sounds ridiculous... but I just can't come up with anything else.

IN a fit of goofiness.... I got a cliplead/copper wire extension....about 4" long...and clipped it on to the tip of the whip.It DID move it down...but ONLY to about 28.3 Mhz with the stinger bottomed out.

At that point.... I quit....before I got to thinking about selling every piece of radio gear I own. I put my toys away, came in here.... to lie down on the "psychologist's couch" so to speak........

Has anyone else had this kind of nonsense go on? The whip is 2" - 3" longer than it should be.... I ADD 4" to it..... and it STILL won't come down under 28.3 Mhz? Did the coax I put on do that?????


I need a drink..........

 

[undertaker]

I always ran the 66 inch whip on my Wilson’s… period nothing else worked!!!

 

[Lkaskel]

I always ran the 66 inch whip on my Wilson’s… period nothing else worked!!!

Where would you get the 66" whip?

 

[midnight special]

magnet mount says it all,,, capacitance is bad i am guessing,,,

 

[brandon7861]

I believe the wilson uses a loading coil inside the base, which would make sense with the shorter whip. Because changing the antenna length affects the reactance more than it does the resistance, changing the antenna length is kinda like changing the loading coil inductance. If I plot a random capacitive impedance (to represent the whip itself), adding a loading coil moves the feed point more along the constant SWR circle than it does toward the center of the chart. This is because the resistance is not 50Ω, it never will be for a short whip. The red dot here is the whip impedance and the green line is what a loading coil did to it.


If the resistive portion of the feed point impedance were 50Ω, the loading coil coulda dropped the dot right in the middle, but its not, it's less than 50Ω (and capacitive). How we go about matching it depends on whether the feed point impedance (without loading coil) is to the left or right of the constant conductance circle passing through 50Ω. If it is to the left, you need a loading coil and shunt capacitor. If its to the right, a loading coil to ground with a tap point can be used.


What I recommend you do is to open it up again, remove the coil and connect the coax directly to the whip, place the antenna where you want it, and then measure the feed point impedance (that means using a half wave coax for the MFJ, unless you can calibrate to the end of the coax with those ones now), and once you know the feed point impedance, we just pop those numbers into the calculator and out comes the matching network you need.

 

[brandon7861]

Just remember, if you are not calibrated to the end of the cable or using a decently made half wave coax, the impedance you measure (and response you see from changing the antenna) could look like you turned my chart sideways. Don't be fooled by a measurement that looks like it is inductive and over 50Ω when using some random coax to take a feed point impedance measurement. Coax rotates that chart around and around and around

 

[Handy Andy]

Would my replacing the original RG/58 coax with a random length of RG/8X be causing this thing to tune high like that???? To ME it sounds ridiculous... but I just can't come up with anything else.

At the risk of starting another "coax length" debate, which is not really needed, but the coax length of this antenna system, is what needs to be taken into account.

Since this is a "mag mount" and where-ever you placed the magnet on the vehicle or shed or panel - you might find that by relocating the mag mount to another spot may change the SWR - if that is the case, then the counterpoise or ground plane you are using - is not developing a good image for that antenna to work with.

Did you try using a known good coax length and type to see if the antenna and it's current whip length - can be trimmed?

This may seen like it - but yes, you are trimming a unknown impedance using a known impedance that provides some reactive components to help cancel out the unknown - and if the SWR goes down - again, relocate the antenna on the panel to see if SWR gets even lower - if it goes up - or down - then you know your loading coil and whip only work to provide a low SWR if you use a specific length of a particular type of coax.

So then this changes the system to be a "tuned unit" and the coax itself acts much like a Balun in this system. It (Coax length) is used to offset the mis-match the loading coil and whip make the system look like to an SWR meter. So it's more of a conjugate match - versus a resonate match. It may appear resonant, but it's only conjugated to show some efficiency into being (appearing as) a 50 ohm load