Dipole Resonance on other bands

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K1BXI:
A brief antennas 101, Jim (from Dallas)....

A regular 1/2 wave center fed dipole will show a low impedance at the center. Also at odd multiples of the resonant frequency it will be low. By low I mean around 80-100 ohms. The even multiples will be very high at the center, a few thousand ohms at times.

So even though the odd and even harmonics are resonant, feeding it with coax on the even multiples is pretty much out of the question. Parallel feed line and a tuner though, will make the regular 1/2 wave dipole work as an all band antenna system. Note the word "system". The "system" includes the wire antenna and the parallel feed line.
 
There is one more thing to consider which is called "end effect" with a wire antenna. This means that the resonant frequency's are not exact multiples of the fundamental frequency. A 40 meter dipole resonant at 7.1 mHz will not be resonant at 21.3 mHz, the 3rd harmonic.  It will be up around 21.9mHz due to this end effect. Think of that center 1/2 wave section as having no ends and the other two 1/2 wave sections on each end only having one actual end. (hope that's clear!)

Simply put Jim, A 1/2 wave dipole will show many resonant points above the fundamental frequency , but very few, if any, that fall within the ham bands will be a direct match to 50 ohm coax. As KZ1X said, "resonance and mismatch are not related in the way I suspect you are considering."

John

(by the time I got my reply typed I noticed the red warning of 3 new replies posted. (A nice feature that I just noticed) Mine pretty much echoes what others have said, so you should have an idea what you can do to achieve what you want.) 

Mel Evans:
Have a look at www.qsl.net/hb9mtn, the indoor antenna might give you some ideas. These are resonant multiple dipoles and there are designs for 80-10 dipoles for hanging outside, just do some Googling, they're there, just have a poke around

Mel G0GQK

Clark McDonald:
Generally speaking, the antenna can be resonant on the third harmonic. 

Since the "original" ham band spacings were set up in harmonics, a 40 meter antenna can be resonant on 15 meters, an 80 meter antenna can be resonant on 20 meters.  etc. 

However, there are other factors to be considered, such as takeoff angle, TANSTAAFL. 


K1BXI:
Quote from: KE3WD on June 19, 2010, 03:05:06 PM

Generally speaking, the antenna can be resonant on the third harmonic.  

Since the "original" ham band spacings were set up in harmonics, a 40 meter antenna can be resonant on 15 meters, an 80 meter antenna can be resonant on 20 meters.  etc.  

However, there are other factors to be considered, such as takeoff angle, TANSTAAFL.  





The ham bands were set up in harmonic relationship so that the 2nd and 3rd etc harmonics of the home brew transmitters would fall in the other ham bands and not bother commercial communications. It has nothing to do with harmonic antennas whose physical lengths are shorter than the electrical length.

It's true an antenna can be resonant on it's third harmonic, but that resonant frequency will be higher and out of the ham bands. A 40 meter 1/2 wave dipole resonant at the very bottom of the band @7.0 mHz will have a resonant frequency of about 21.700 mHz, a little high and out of the the band. That's why Jim, N2EY mentioned those added wires to help bring it back down a bit. Your 80 meter 1/2 wave dipole will show resonance @ around 15 mHz and since it will be fed at a voltage point it will have a very high input impedance. Not what the poster was looking for.

The formula for the length of a harmonic wire antenna is 492*(N-0.05) divided by the frequency in mHz. N being the number of half waves on the antenna. An example is the 102' G5RV, 3 half waves @14.250 mHz. Yet a 1/2 wave dipole at that frequency is just under 33 ft. Three of them lined up end to end would be just under 99 ft (98.5 to be exact).

Now if he wants to use a closed loop antenna he may have somewhat better luck in doing what he wants. In any case, some bands will still require the use of a tuner of some sorts.

John

 

Dale Hunt:
First, others have noted that "completely resonant" and "low enough SWR to use without a tuner" are
NOT the same thing.  I am presuming you mean the latter.

Then we also need to know what sort of rig you are using.  Some of mine don't complain at all when
the SWR is high.  Some are happier at 100 ohms than at 25 ohms, even though the SWR is 2 : 1 in
both cases.  The older tube rigs generally had a much wider matching range (especially on the higher
bands.)

Just starting with a half wave dipole, the most common combination you will hear about is 40 / 15m.
As has been mentioned, the resonance is generally above 15m and the feedpoint impedance may
be above 100 ohms, giving an SWR over 2 : 1.  This worked fine with a lot of the tube rigs (though
the feedline length might have to be tweaked a bit) but generally requires capacity hats or
other modification to work with the modern solid state rig that limits itself to a 2 : 1 SWR.  And even
at that, the best SWR you can get may be close to, or over, the limit.

The only practical combinations will be the 3rd, 5th or 7th harmonics.  That leaves us with very limited
possibilities for the HF bands: the fundamental has to be below 10 MHz for the third harmonic to
be below 30 MHz, and below 6 MHz for the 5th to be usable.  So, what can we do with 80m?

Using EZNEC to model it, a dipole cut for 3.5 MHz (at 40' using #12 wire, since these things may
matter in the final solution) shows SWR dips at 10.9 MHz (SWR = 2.5 : 1), 18.15 MHz ( 2.5 : 1 ) and
25.5 MHz ( 2.5 : 1).  With some coax loss to lower the SWR at the rig and you should be able to
use it on 17m, but it is still too short for 12m.  The same dipole cut for fundamental resonance
at the top of the 80/75m band (4.0 MHz) gives similar SWR readings at 20.9 and 29 MHz:  still
a bit short to cover 15m, but usable on 10m.  Setting resonance about 3.9 MHz should give
coverage of the 28.3 - 28.5 MHz portion of 10m, but again with best SWR around 2.5 : 1.

Things like height above ground, nearby objects, using an inverted vee rather than a straight dipole,
etc. will vary these: some combinations might have a lower SWR in some situations, some may be
higher.  There are many other ways to make multi-band antennas, but these are the only likely
combinations for coax-fed HF dipoles.

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