Survivalism & Self Sufficiency Topics > Amateur Radio Gear Reviews

MFJ-207 Antenna Analyzer

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Carl:

--- Quote from: GreekMan on March 22, 2015, 02:18:55 AM ---qot a question...is that -5% peculiar to the inverted V dipole?

--- End quote ---

It is commonly accepted,but alway cut the wire a bit long,because it is really hard to add wire,
and just fold the wire back on itself after it is attache to the end insulators....folded back like
this the extra wire is not effecting the antenna tune.

armymars:
  The antenna looks longer because of the interaction of the ends to the ground. This adds some capacitive reactants to the antenna.

Greekman:
hmmm thansk for that....

it may be the qause why my wire 1/4 grounpalne seemed also longer when I was tuning/cutting it 3 feet of the ground only...

Smurf Hunter:
I have used my internal tuner and swr tuner to locate the frequency my wire has the lowest SWR on.

Is that same frequency ever NOT also the most resonant?

e.g. a dipole shows lowest SWR at 14.3mhz

I know my radio will get the most from the TX watts on 14.3.  What other data could help?

Carl:

--- Quote from: Smurf Hunter on March 24, 2015, 09:10:15 AM ---I have used my internal tuner and swr tuner to locate the frequency my wire has the lowest SWR on.

Is that same frequency ever NOT also the most resonant?

e.g. a dipole shows lowest SWR at 14.3mhz

I know my radio will get the most from the TX watts on 14.3.  What other data could help?

--- End quote ---

Resonance is loosely associated with SWR though not as one might think.
How resonant is a DUMMY LOAD?   Yet it has ,or should have ,low SWR.
You don't TUNE an antenna by cutting or adding length...you tune the SYSTEM
to make the radio happy...nothing more.

Please read here ...again.

http://www.qsl.net/arrlsb/Digest/Pages/Antennas/antennas03.html

A quote from the article:

The Basic Half Wave Antenna
By “antenna” I mean the antenna itself and excluding the feedline attached to it. The first misconception hams have is that the closer the basic dipole antenna is to a half-wave length in the band of interest the closer it will approach perfection and the better it will radiate.

That belief is not supported by either theory or practice. Theoretically, the half-wave antenna does not radiate any better or worse than any other length. In other words, all lengths radiate equally well. You will not find any statement to the contrary in any reputable antenna text including the ARRL Antenna Book. Nor will you find any claim that the half-wave antenna maximizes radiation compared to other lengths.

So why does every antenna article ever published show the half-wave dipole antenna as the basic antenna? The answer is because it is the basic antenna. But this does not mean that it is the better antenna. In text books it is the basic antenna because it is the easiest length by which to describe mathematically and conceptually how antennas in general operate. For one thing the sinewave-like distribution of voltage and current along the wire can be drawn very nicely to exact fit on the half-wave wire. Can you imagine what it would be like if your basic antenna text book began by describing a 37/64 or an 9/17 wavelength antenna? It can be done, but the drawings and the mathematics would really by messy using such odd lengths. But a half-wavelength...well, how elemental can you get?

But why is almost every real-world antenna a half-wave-length long? Now we’ve come to the crux of the matter. The answer is because, in the practical world, the no-tune transmitter wants to see 50 ohms and the coax cable wants to see 50 ohms and the half-wave dipole impedance can be made close to 50 ohms. Everything comes out closely matched and everybody is reasonably happy. It is the half-wave-length antenna that makes all of this possible. This convenient accommodation to the demands of the transmitter, however, says nothing about the radiating efficiency of the antenna itself. Nor does it make this antenna length the best radiator. The only thing it says is that this length makes the SWR low at the transmitter. SWR describes a condition on the feedline. SWR says nothing about the radiating qualities of an antenna.

That a low SWR does not validate a good antenna is easily demonstrated. Consider a 50-ohm resistor terminating your feedline. The SWR is a perfect 1:1 but would you use that resistor as your antenna?

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