[TowerTalk] TH6 SWR on 10m

[Jim Smith]

Using an ordinary ohmmeter to measure low resistance values can be
problematic as you are often unsure how good the connection to the
ohmmeter is.  A technique I used a few months ago to check the dc
resistance of all the joints in an old TH3 is as follows:

1. Connect the ends of the element to be tested to a DC Current
Source and adjust the current flowing to 1 Amp.
	I used an old Heath LV power supply which has adjustable current
limiting.
2.  Connect a DC voltmeter between the ends of the elements.
	Connect directly to the elements, not to the power supply wires.
3.  Read the voltage.

This number equals the total resistance of the elements and all
the joints.  (Ohm's Law strikes again.)
To find the resistance value of any one joint, just measure the
voltage drop across it.
	In my case, the voltage drop of each joint was less than 0.1
millivolt as measured on my el cheapo RS pocket DMM.  This means I
was able to confirm that the joint resistances were all less than
one tenth of a milliohm, i.e. less than 0.0001 ohms.  Pretty tough
to do this with a regular ohmmeter. 

BTW, this antenna was assembled in 1967. 


> 
> Depending how long it's been up I'd take the whole beam down and disassemble
> the traps, clean every joint and regrease.  Most hams are under the total
> allusion that joints remain the same after a time.  I check them with a HP
> low resistance ohmmeter.  A conventional ohmmeter is often all you needed.

Hope this helps.

73 de Jim Smith	VE7FO

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