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Author Topic: Simple antenna question  (Read 656 times)
M5AEO
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« on: February 28, 2013, 01:12:25 PM »

Yesterday I put up a 'lazy' dipole, in other words it had to droop a bit in a strange shape (I only have my appartment balcony for space!).  I cut it to work on 40m, so each leg was about 33 feet.  I found that it also worked well on 15m, but not on any other band, even with a ATU.  Can someone just remind me about the relationship between these two bands in terms of wavelength and antenna efficiency?

Jonathan M5AEO, London.


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N3OX
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« Reply #1 on: February 28, 2013, 01:21:19 PM »

Something to keep in mind about 15m is that the wavelength is actually between about 14.3m and 14m, while the 40m band actually lies between 42.8m and 41m.

A 1/2 wavelength dipole for the "40m" is about 3/2 wavelength on the "15m" band. Of course you can see this from the frequencies too.. the third harmonic of the 40m band from 7.0-7.15MHz falls inside the 15m band.

A resonant 3/2 wavelength doublet tends to have an impedance that's a pretty decent match to coax. There are some differences in the exact resonant frequency of a 3/2 dipole compared to a 1/2 wavelength dipole such that you might have a *really low* SWR on 15m from a 40m dipole, but what you WILL tend to get is an impedance that doesn't mismatch the coax HORRIBLY, so you won't get much extra coax loss because of a mismatch.

It's not really an issue of the efficiency of the wire itself, but an issue of increased feedline losses on most bands if you're feeding with coax, but not bad losses on 15m.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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« Reply #2 on: February 28, 2013, 02:12:42 PM »

An antenna like a dipole will have a low feedpoint impedance (suitable for
use with coax) when the length on each side is an odd multiple of 1/4 wave.

On 40m each wire is 1/4 wave.

On 15m each wire is about 3/4 wave.

If you have an SWR analyzer you should find another point of low SWR
around 35 MHz where each side of the antenna is 5/4 wavelengths.

Meanwhile, on 20m it is 1/2 wave and a full wave on 10m, so those bands
will have a very high impedance, hence high SWR.

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K5LXP
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« Reply #3 on: February 28, 2013, 02:35:31 PM »

If it had a dip on 15M, your 40M dip is probably below 7MHz.  See if you can find it below the ham band.


Mark K5LXP
Albuquerque, NM
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