Ed wrote in
92.196:


This is the first time I've done this. If the need arises again,
I'll
have to download and try EZNEC, I guess.


Meanwhile, I am concerned about the efficiency of our ARES Command
Center HF antenna system.


Could someone please advise me what feedpoint SWRs would be seen
on a
wire antenna, 15 feet above a flat metal roofed building, (it a NVIS
antenna), and resonant at 6.2 MHz.


It is operated at 3.98 MHZ, 5.4 MHz, and 7.2 MHz


The 100 watt radio has one of those built in tuners that can only
handle 3:1 SWR. I suspect we are terribly inefficient.


Ed,

I am not sure exactly what you mean by a "a wire antenna, 15 feet above a
flat metal roofed building, (it a NVIS antenna), and resonant at 6.2
MHz".

I assume you mean a centre fed dipole that is an electrical half wave at
6.2MHz.

The efficiency of the radiator component can be estimated from an NEC
model, it is probably very high and not the real issue.

Components of an antenna system interact with each other in a complex
way, and it is important to analyse the entire antenna system (radiator,
earth, transmission line, balun, ATU etc) to obtain a correct
understanding of how the antenna system works overall.

In your case, the feedline and ATU are the likely main contributors to
antenna system loss. Antenna system loss, transmitter behaviour, and
antenna pattern are the main contibutors to station performance.

To illustrate with an example. One Saturday a few months ago, I had 7MHz
three QSOs in a row with our new six hour hams who were each using an 80m
half wave dipole fed with a substantial run of RG58C/U coax and an ATU. I
explained to them that their antenna would work ten times better on 7MHz
if their antenna was half the length. A difficult concept for people with
six hours investment in ham radio to understand. A fourth person who had
heard one of the QSOs (and had apparently called me, but I couldn't dig
him out of the noise) emailed me saying he was using the same setup and
now knew the problem, it had to be the ATU. I replied to him that the ATU
probably wasn't losing all that much power, the high coax losses
protected the ATU from an extreme load and extreme loss. Again a hard
concept to swallow when the ATU is closer to the transmitter but
something downstream "protects" it from higher loss.


Owen