OK, I take that answer...now, why did the FCC go with an ERP as that
would require calculation rather than a measurement. Hard to measure
ERP as far as I know...

Oh well, go figure!

Scott
N0EDV

Dave wrote:

Scott wrote:

So, hows does an armchair operator (one who doesn't have $10K worth of
test equipment) determine the efficiency of the antenna?

Scott
N0EDV

SNIPPED

There are two easy methods and one rule of thumb method that will get
you into the ballpark.

1) As Cecil replied, divide the radiation resistance by the feedpoint
resistance. EZNEC will give a reasonable value for radiation resistance
and an MFJ 259B [~$250] will give a measure of the feedpoint resistance.

2) A freeware program, mobile antenna, will also calculate the efficiency.

3) [Rule of thumb] The gain is proportional to the effective aperture in
square wavelengths. So, as mentioned in another reply, approximately,
the ratio of length of the mobile antenna [~8 feet] to the length of a
1/2 wavelength antenna for 60 meters [~93 feet] yields 8.6% efficiency.
So, 100 watts from an IC-706 [series] yields ~9 watts effective radiated
power. Conclusion, 50 watts ERP on 60 meters with a 9% efficient antenna
would require 555 watts into the antenna.

/s/ DD

On Feb 23, 7:57 pm, Scott wrote:
OK, I take that answer...now, why did the FCC go with an ERP as that
would require calculation rather than a measurement. Hard to measure
ERP as far as I know...


Because ERP (or EIRP, depending on the application) is how most
licensing works, because that's what's important for spectrum
management in a shared band. Ham radio is somewhat unique in that the
rules are applied on power somewhere in the middle of the system (i.e.
in the feedline from Tx to antenna), with no limits (except for RF
safety) on the radiated field strength. If you can build a 60dBi
antenna and radiate 1.5 GW EIRP, you can do it. (part of that
'encouraging experimentation and advances in the radio art').

yes, commercial broadcast is regulated in terms of power output, but,
since you have to do a "proof of performance" and demonstrate that you
have a certain field strength at a distance, it's really regulating
ERP. Change your antenna gain, and they can conceivably make you
change your Tx power.

On Sat, 24 Feb 2007 03:57:24 +0000, Scott
wrote:

OK, I take that answer...now, why did the FCC go with an ERP as that
would require calculation rather than a measurement. Hard to measure
ERP as far as I know...

Hi Scott,

This never intimidated the FCC when they handed me tests for
RadioTelephone Second and First Class. In this group, we can account
for ERP by using modeling tools such as EZNEC. This and other
modeling tools generally agree with measurements found in the field.

73's
Richard Clark, KB7QHC