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VHF Groundplane Measurements

'Inspired' by the many assumptions, speculations and absolute 'gospel' claims that a 1/4 wave VHF antenna makes a great 3/4 wave UHF antenna - because of the "3:1 ratio" - and I used to think so until study, science, measurements and experience proved otherwise...

I set out to do some simple tests and illustrate the results with an all-around 19" whip/radial groundplane.

The results do not bear out as well as the claims over either wide ranges nor at the most common amateur transmitting ranges.

Receive is seldom a qualified measure because no one does it. What matters for a GOOD antenna is a proper match, at frequency/resonance and match, almost regardless of SWR, for your transmitter - the expensive part.

Also disregarded, but modeled by EZ-NEC by others, is the radiation pattern. An ideal quarter-wave pattern is like a big round ball. A longer (vertical) antenna typically compresses or lowers the pattern due to RF-wave phase characteristics.

Best VHF SWR = 1.2 @ 145, best VHF 50 ohm matches @ 143 and 148 with SWR of 1.3 (not bad!)

Best UHF SWR = 1.3 @ 450 MHz, SWR at 439 = 1.9, SWR at 445 = 1.9. (no impedance available with analyzer)

* 145 x 3 = 435 so per the measurements the coincident best SWR does not occur at or bear out the 3X myth.

If you made the antenna shorter to bring VHF SWR down at a more common 146 or 147 MHz, you would raise the UHF best SWR frequency into the business band, and start to compromise lower 440 ham effectiveness.

This gets MUCH worse when you try to decouple ('center-load'?) a 1/4 wave for dual band to try to bring the impedance in line, worse with a longer loaded gain dual-bander... another test to run to illustrate, but see the mini-mag album for an idea.

On to the pics...

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