I am seeing posts indicating that some are equating a 1:1 SWR with resonance
and attempting to adjust the antenna/radial/ground tuning on the basis of
SWR... The two items are separate issues...
1... Resonance (1st harmonic) is the point of lowest Z at the antenna
terminals when measured by your RF-1... This can be validated by using a noise
bridge as a cross check... It is rare (or blind luck) for Z to turn out to be
50 ohms...
2... SWR at resonance may or may not be 1:1... If the feed point approximates
50 ohms you will see something approximating 1:1 on a nominal 50 ohm bridge...
If the feed point is in the vicinity of either 25 ohms or 100 ohms, then you
will see an SWR approximating 2:1... and so on...
3... At frequencies away from resonance you will have reactance, expressed as
+/- J , which affects the SWR bridge reading... There are combinations of J
and Z which can make the SWR bridge look pretty good, while the antenna
itself is hoplessly mistuned... Which is why you always have to go back to
basics, get the antenna resonant, and THEN deal with matching the feed line to
the load to get a SWR reading that keeps your 3CX3000A7 Godzilla Stomper
happy...
Does the antenna HAVE to be resonant?
Of course not... There are many nonresonant antennas in use, a shunt fed tower
being a common example... But a mismatched antenna/feedline will have lots of
circulating current in the feedline, and a highly reactive feed point in the
shack can make life miserable... High circulating currents can cause
significant power loss in the feedline, although ladderline works pretty well
in such situations....And you have to match the feedline to the transmitter in
the end, anyway... So why not do the matching at the antenna, and if you can
make the antenna resonant in the process so much the better for useable
bandwidth...
On the question of tuning insulated ground radials, either 1/4 wave, or loaded
1/8 wave...
Of course you can tune them... They will be physically shorter than if they
were elevated... For those who say that laying on the ground totally detunes
them, I have a proposal... Touch the far end of one of the radial wires to
your tongue and have a friend (fiend?:) key up the rig - about ten watts
should do it nicely... (Also, scoffers may review the literature on
underground/submerged antennas as a less painful method of enlightenment :
)... Ground proximity drastically lowers the impedence to the point that
standard bridges are unuseable for setting the resonance point... I suggest
that you configure a pair of the radials as a dipole - elevate them about 3
feet (which will bring the impedence up to where you can measure) and use your
RF-1 or noise bridge to adjust the length to resonance in the middle of the
band, then cut all the radials to that length... Putting them back on the
ground will pull the resonant frequency down another few percent, just about
where you want it...
Do you have to tune them?
Nope... I will bet that most ham ground radial systems are untuned... and most
of them work, though some work better than others...
--
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