Is a 1/4 wave 6m antenna also resonance for 2m.

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BJ:
20 inches does not equal 1/4 wave on 6 meters in my book.

Using the 234 formula a quarter wave 6 meter antenna should be 52 to 54 inches long.

Not sure what kind of antenna you have.

dennis smith:
That is 20 feet off the ground. The 1/4 wave 6m vert part is 55 inches

Dale Hunt:
Probably not.

The resonances will be roughly odd multiples of the design frequency,
so an antenna designed for 52 MHz will probably resonate close to
156 MHz.  (Actually I tried one on EZNEC and it came in at 159 MHz.)

Some inductive-loaded 5/8 wave whips for 2m will also resonate on
6m, however.  That is because the effect of the coil is different on the
two bands.  That is a reasonable approach for mobile use, but not
necessarily for fixed use with a ground plane (which messes up the 2m
pattern anyway.)

The main problem is that, even if you can load the 6m antenna on 2m,
the radiation pattern won't be ideal.  A 3/4 wave vertical has a lot of
radiation at high angles.  A shorter 1/4 wave antenna probably will
put out a stronger signal at the horizon.   This is a problem when any
vertical antenna is made too long, unless some method is used to change
the relative phase of the RF between adjacent half wave portions of the
radiator.

One simple solution might be to add a 1/4 wave element for 2m at the
feedpoint in parallel with the 6m element.  You might need to add a
couple of 2m radials as well.

Lon Kinley:
Dennis:

How's the work on the license coming?

73,

Lon - W3LK
Naugatuck, Connecticut

Bob Lewis:
The formula for 1/4 wavelength is:
 L (in feet) = 234 / f (in MHz)
 L (in inches) = (234 x 12) / f (in MHz)

So a 1/4 wavelength on 52 MHz is 54 inches. A 1/4 wavelength on 146 MHz is 19.2 inches. Thus a 1/4 wave 6M antenna is approximately 3/4 wavelength on 2M, depending on exactly where in the bands you are operating. The 3/4 wavelength antenna should be a fair match for 50 ohm cable but the radiation pattern (take off angle) is going to be pretty high (ideal is near the horizon).

Two things are required for a direct coax feed on an antenna. First it should be resonant (i.e. zero reactance). Second it needs to be close to 50 ohms for minimum reflected power on the feed line. The two things often don't happen at exactly the same antenna length. For example, an end fed antenna that is 1/2 wavelength long is resonant but its impedance may be a couple thousand ohms resulting in a VERY high SWR on 50 ohm coax. On the other hand an end fed antenna that is 1/4 wavelength will be much closer to 50 ohms and usually results in an acceptable SWR on the 50 ohm feed line. An end fed antenna that is an odd multiple of 1/4 wavelength (like 3/4 wavelength) has a low impedance feed somewhere around 50 ohms.

73,
Bob

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