View Single Post
Old 16-Nov-2014, 7:00 PM   #2
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,569
I have bought some miserable F81s. Some don't grab the center conductor very well; others don't even allow the center conductor to enter!

I test every one before I use it by inserting a short length of 18 gauge solid copper wire to see how well it grabs it. That's the same gauge as the center conductor of RG6.

Here I'm measuring the output voltage of the CM0747 power supply/inserter under load. It is necessary to have access to the center conductor to measure the voltage and current, hence the 18 gauge hookup wire:


The photo is from this post:
http://www.avsforum.com/forum/25-hdt...l#post20502719

from this thread:
CM7777 preamp failure question
http://www.avsforum.com/forum/25-hdt...-question.html

Quote:
Can they cause signal loss in any circumstance?
The signal loss is usually less than 0.5 dB for UHF TV frequencies, but it becomes important for satellite frequencies in the GHz range. Can I assume you are talking about frequencies no higher than UHF TV?

The better ones are rated to 3 GHz:
http://www.mjsales.net/itemsearch.asp?FamilyID=854

These are rated by pin holding force:
http://www.3starinc.com/manuals/Holland-F-Splices.pdf

These have a current rating of 8 amps, which is probably more than you need for a TV preamp in the 100 to 300 mA range:
http://picodigital.com/product-details.php?PID=156

Using the Right F-81 Connector
http://www.tselectronic.com/tech_notes/right_f81.php

Barrel connectors for satellite discussion:
http://www.dbstalk.com/topic/189731-barrel-connectors/

The current carrying ability is determined by the contact resistance, as it is in switches. You can measure the contact resistance by using what is called a 4-terminal resistance measurement which uses a constant current and a measurement of the voltage drop. Ohm's law then gives you the contact resistance.

The simple way is to measure the voltage delivered to the preamp to see if it is sufficient for proper operation. This becomes more important with long runs of coax. RG6 with a solid copper center conductor has less DC voltage drop than RG6 with a copper coated steel center conductor. The TV signals use just the surface of the center conductor; this is called the "skin effect." The DC current for the preamp power uses the whole cross-section of the center conductor.
Attached Images
File Type: jpg CM0747test2_1.jpg (47.0 KB, 853 views)
__________________
If you can not measure it, you can not improve it.
Lord Kelvin, 1883
http://www.megalithia.com/elect/aeri...ttpoorman.html

Last edited by rabbit73; 16-Nov-2014 at 11:15 PM.
rabbit73 is offline   Reply With Quote