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Author Topic: Matching a ground-mounted mobile whip  (Read 2100 times)
K5BJS
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Posts: 50




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« on: February 27, 2012, 03:43:33 PM »

After reading about the FX Portable Antenna, I was inspired to try something similar with an MFJ 40m HF Stick.

Assembled the antenna this weekend, and I'm measuring about 17 ohms at resonance.  How should I go about matching it to 50 ohm coax?  Is there such a thing as a 3:1 unun?
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WB6BYU
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Posts: 17046




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« Reply #1 on: February 27, 2012, 04:02:24 PM »

For 40m, connect a coil across the base of about 0.8uH, or a 560pf or 680pf capacitor.
Readjust the whip for minimum SWR.

Or use a quarter wavelength of 29 ohm coax.  (Equal electrical lengths of 75 ohm and 50 ohm
coax in parallel would get you pretty close.)

Or a 4 : 1 step-up transformer and 75 ohm for the feedline.

If you have a loading coil at the base, try winding a few turns of wire around that and
connecting it across the coax.  Adjust the number of turns on the link winding for the
desired impedance and resonate the antenna by adjusting the loading coil.

There are a number of other methods, but these are probably the simplest.
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KH6AQ
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« Reply #2 on: February 27, 2012, 06:11:43 PM »

I use a ground mounted screwdriver mobile whip and match it using a DX Engineering DXE-MM-1 transformer. It will match 12.5 or 25 ohms to 50 ohms. I run it at 1200 watts and it hasn't burned up yet.

Your 17 ohms base resistance along with the 2.5 ohm radiation resistance of this antenna means the radiation efficiency is 15%. What kind of ground system do you have?
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K5BJS
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« Reply #3 on: February 28, 2012, 05:42:24 AM »

Quote
Your 17 ohms base resistance along with the 2.5 ohm radiation resistance of this antenna means the radiation efficiency is 15%.

Are typical values of Rr and Rc for these antennas published somewhere?  If so, you could calculate Rg by measuring Rt.

Quote
What kind of ground system do you have?

Twelve 16.7 foot radials at the moment.  It's entirely possible I didn't measure the base resistance correctly.  I used a 2 foot jumper between the analyzer and the antenna, so I was sitting pretty close to the antenna when I took the measurement.
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