[ICOM] ICOM 718 SWR meter

[Larry Young]

Patrick: The only time your antenna analyzer will correctly read your antenna impedance is when connected directly at its feed point, or through a piece of 50 Ohm coax that is an electrical multiple of 1/2 wavelength that is connected at the feedpoint. Unless the antennas feed point impedance is 50 Ohms (SWR 1:1), on any antenna impedance other than 50 Ohms, the coaxial cable itself will act as a stub or transformer, giving an erroneous reading. It is highly unlikely that your vertical antenna has an impedance close to 50 Ohms, Tune your antenna to resonance (where the inductive and capacitive reactances cancel, not necessarily the lowest SWR) then improve the radial system for the lowest impedance to improve the antenna efficiency.
Larry K4LXV.

--- On Sat, 2/26/11, Patrick Schwarz - KB8RTZ <kb8rtz at oh.rr.com> wrote:

From: Patrick Schwarz - KB8RTZ <kb8rtz at oh.rr.com>
Subject: [ICOM]  ICOM 718 SWR meter
To: icom at mailman.qth.net
Date: Saturday, February 26, 2011, 7:32 PM

Curious if anyone else has checked the SWR meter built into the IC-718 
with the SWR meter of an antenna analyzer such as the MFJ-269?

Here is the setup:
Antenna - Hustler 6BTV
Analyzer - MFJ-269

Process of tuning the antenna prior to purchasing the antenna analyzer:

Connected a 25 foot feedline (RG8U) between the antenna and transmitter. 
Per the antenna manual, tuned the antenna on 10m, 15m, and 20m. 30m on 
down did not tune well or at all. Ground radials helped, but not enough 
below 30m. Came to the conclusion that my tuning skills left a lot to be 
desired so invested in an analyzer.

Process of tuning the antenna with the antenna analyzer:

First, reset the trap lengths to factory lengths, then overall tube 
lengths to suggested starting lengths using ground radials. Connected a 
9 foot feedline (LMR-240) between the antenna and analyzer, manually 
tuned the antenna per the manual. Disconnected the analyzer and tuning 
feedline from the antenna.  Reconnected the transmitter feedline (this 
time a 75 foot feedline (RG8U)).

Checked the SWR at the transmitter feedpoint with the analyzer. The 
results at the antenna with the tuning feedline and at the transmitter 
feedpoint were comparable. Disconnected the analyzer and reconnected the 
transmitter. The SWR meter built into the IC-718 measured relatively 
close results on 10m and 15m. The SWR on 20m was a little higher than 
measured by the analyzer, higher yet on 30m and 40m, and worse yet on 
75/80m (granted, 75m is not fully tuned yet).

Seems as though one or the other is incorrect. The SWR at the antenna on 
say 7178KHz on the MFJ reads 1.6. The SWR at the transmitter feedpoint 
using the MFJ reads 1.6. The Icom SWR meter reads 2.1 or so requiring a 
little tuning.

Has anyone else noticed these types of results?

Thanks,

Patrick...KB8RTZ...
----
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