At 01:49 PM 8/3/2006, Keith Dutson wrote:
> >As I understand it that just makes the radio happy. What is the ERP
> >delta between a resonate antenna and a non-resonant tuner-tuned
> >antenna?
>
>Essentially identical.
>--------------------------------------------------------
>
>Are you making some assumptions here?
Yes, I was assuming that the resonant and non-resonant antennas were
"about" the same length and installed in "about" the same place.
>What is I have a perfectly resonant
>coax fed dipole at 3850 KHz at 140 feet and run 200 watts CW. Now compare
>this to a 30 foot long piece of wire at 20 feet fed directly with a tuner at
>the same frequency. Is ERP affected in this case?
ERP (but not EIRP) would be the same (except for copper losses in the
shorter antenna). Gain would be different (the dipole is 2.15dBi, the
short wire, closer to 0dBi, because the pattern would be more
"omni"). And, there would be whatever effects you have from running the
antenna close to the ground.
Put that 30 foot piece of wire up at 140 ft with the tuner, and then the
comparison is a bit more fair. The current will be higher in the 30 ft
piece of wire, as opposed to the 130 footer, so IR losses will be
higher. And, if you compared at a higher frequency, where the coax losses
are higher, you might wind up with the lower antenna actually being more
efficient (in a watts radiated compared to watts out of transmitter) than
the higher one.
But, for basically the same physically sized antenna, whether it's resonant
doesn't markedly change the gain or pattern, just the impedance at the
feedpoint. So, if you have that 3.5 MHz antenna, and it happens to
resonante at 3.4 MHz or 3.6 MHz, the pattern and efficiency will be about
the same, assuming your matching network is reasonably low loss (and at the
feedpoint).
If you're depending on the particular resonance (as in a parasitic array,
like a Yagi), then that's another story, because the different tuning
changes the phase.
Jim
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|