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This time there will be something about "problems" with antenna measurements ...
Sometimes I receive questions like that:
" Why does the MAX measurement give result different from the measurement of SWR meter built into the TRX "
or
" Why does the measurement of the MAX in LCD mode show something else than the measurement visible on the PC "
Often the explanation of this fact is simply bad antenna - easy to say - worse to improve .
Several measurement principles :
- Measurements are better to be started in LCD mode
- Work with the built-in battery
- Do not use the AC adapter
- Do not connect the cable to your computer
- Keep the meter for plastic housing
- Do not touch the antenna weight or metal screws
- Antenna cable from the antenna not connected to "ground bus" in hamshack
Similar rules apply when doing measurements directly at the antenna feed point
Then we can make this test - that is,
1 - we measure - we see SWR quite well :-)
2 - touch the antenna ground by hand ...
3 - touch the antenna ground using antenna cable in shack
3 - touch the antenna cable weight mass of shack
( As for the cable that goes into our shack )
If the antenna is well constructed - the measurement has no right to change or OFFSET . Stay look this same on LCD with no any reaction to TOUCH
If the antenna does not have a good counterbalance or under the radiators / radials for bad angles , or other construction issue is when you touch the GROUND by your hand or cable ( your hand becomes a part the antenna) - we can see changing measurements on LCD !
The same applies to the case :
- FIRST - we measure in LCD mode
- SECOND - we measure with a computer ( MAX6 + PC)
so . ..... and our measurements in both case looks different - just simply a computer has become a piece of antenna ...
Only if we know that the tested antenna is the REAL FINISHED ANTENNA and not " incomplete radiator " we can easily connect to a PC or TRX and be sure we have good antenna.
Working with a laptop on battery operation is similar to an "isolated " :-) like in LCD mode.
But better first measurement - if this is possible - perform on antenna feed point.
A few words of explanation on the example of well-known CB antennas on a magnetic base - a common problem which is a magnet for small capacitor to the body and another measurement will be how we measure ;
- MAX6 in hand ... cable to antenna in random position
- MAX6 in hand - cable laid close to metal cabinet of CAR ( more capacitance)
- MAX6 in his hand and we touch GND to the car ground
.... If measurements are changing - Antenna is for repair or replacement (eg lack of ALU foil in the base or no contact with the ground foil cable in base )
Here are the photos of the construction of GP helical band 80 meters ( HELIKAN )
- Initial design was right on the ground radials - nicely clothed
- New version had power raised to 1.8 meters and radials mounted obliquely
- That antenna nicely tuned and X went to zero it was necessary to move close to ground all radials
In the end, the measurement of the antenna does not react to touch the cable that went to TRX ... and just as it appeared on the measurement of 40 m cable end ;-)
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