[TowerTalk] Using Stubs to Reject Harmonics

[TexasRF at aol.com]

Jim, you are right, it was a 200 ft length for 432 MHz and 100 ft for  
higher frequencies. This is a great application for old RG8 coax that has gone  
bad.
 
200ft of new RG213 measures right at 4.5 dB/100ft at 432 MHz so 200ft is  
almost 9 dB and a return loss of 18 db with short or open at the far end. I 
have  a couple of calibrated for loss 100 ft cables that I use for 
attenuators for  these higher frequencies. They allow power measurements using lower 
power Bird  slugs which seem to be more accurate than the 2500 watt slugs I 
have used,  especially at 1296 MHz.
 
Of coarse the Bird meter is terminated with a smaller 200w load when used  
this way and the return loss looking into the "attenuator" cable becomes  
more like 30 to 40 dB.
 
73,
Gerald K5GW
 
In a message dated 2/4/2010 2:50:49 A.M. Central Standard Time,  
jim at audiosystemsgroup.com writes:

On Wed,  3 Feb 2010 20:24:47 EST, TexasRF at aol.com wrote:

> For example, a  100ft length of RG213 has about 10 dB loss at 432 MHz

That would be  some pretty awful coax -- Belden's RG8X (9258) is only 
6.6dB/100 ft at 400  MHz, and a decent RG8-sized coax is on the order of 
3dB/100 ft.  

Yes, an infinitely long line looks like a resistor equal to its own  Zo, 
even with an open circuit as a load but it takes more like 1,000 ft to  
approach that at 432 MHz. 

73,

Jim  K9YC


_______________________________________________



_______________________________________________
TowerTalk  mailing  list
TowerTalk at contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk