message posted in rec.radio.amateur.antenna
It confirms that there can be problems with the accuracy of the dummy
load, but the final paragraph suggests that factory calibration problems
are also a possibility.
------- Forwarded message follows -------
I own a 259 but had trouble getting various antenna's tuned to speciic
frequencies using it. Although the 259 indicated the antenna was at 50
ohms and had a satisfactory SWR. So, first I measured the impedance of
my dummy load using a Fluke meter--the dummy load read 56 ohms. I went
through 5 other dummy loads from other locals--ranged from 44 ohms to 58
ohms. I finally found a 49.8 ohm dummy load. When I checked the MFJ
259 against the that dummy load it read 40 ohms. So I calibrated the
259 to the dummy load (49.8 ohms) and it started to indicate properly
based on antenna performance. So my lesson learned was:
1. Check the impedance of your dummy load, you might be suprised.
2. Calibrate your MFJ at 4 MHZ (factory recommendation) into a dummy
load.
3. Take off the cover and look for three pots in line on the board,
horizontally positioned, the middle pot will allow you to set the proper
impedance reading as indicated on the meter. Make sure your dummy load
is close to 50 ohms.
Have done 5 calibrations and they all seem to work
better than the factory set position. One was off by 20 ohms.
73's Rick, W7AV
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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