[Top] [All Lists]

Re: [TowerTalk] Using Stubs to Reject Harmonics

Subject: Re: [TowerTalk] Using Stubs to Reject Harmonics
Reply-to: "Tower and HF antenna construction topics." <>
Date: Wed, 3 Feb 2010 20:24:47 EST
List-post: <">>
Hi Paul, I think you are confusing impedance and resistance here. There is  
an infinite number of resistance and reactance combinations that will equal 
50  ohms impedance but the only ones with low vswr are the ones at or near 
50  +j0.
Interestingly, a coax long enough to have substantial loss does make a good 
 dummy load. For example, a 100ft length of RG213 has about 10 dB loss at 
432  MHz. With a short or open circuit at the far end, the reflected wave 
will be  attenuated 20 dB (this is what 20 dB return loss is all about). A 20 
dB  reflection is equivalent to 1.2 vswr. When the length is doubled, the 
return  loss is 40 dB and vswr is like 1.02 or so.
I use a length of RG213 like this for a dummy load on 432, 1296 and 2304  
MHz with excellent results. The only caution is that the coax gets really hot 
 when it is absorbing 1500 watts power at these frequencies. After just a 
few  minutes there is a smell of burning plastic and you know it is time to 
let the  cable cool a bit.
Maybe a bucket of water would take care of that but I have never tried  
that. Taking all this one last step, one could monitor the temperature rise of  
the bucket of water to make a poor ham's calorimeter for precision power  
Gerald K5GW
In a message dated 2/2/2010 7:50:04 P.M. Central Standard Time, writes:

>  Which is why W2VJN's stub cookbook shows some interesting behavior
>  with 1/8 wave connecting lines <G>.

Using TLW a while back, I was  modeling a 1/8-wave line and it looks like 
matter what the termination  Z at the end of 1/8-wave line, the Z seen at 
transmitter is always the  line Zo. Depending on whether the line is open, 
short, or somewhere in  between, the magnitude of reactance varies greatly 
does the line SWR,  but the Z seen by the transmitter will always be fixed 

Hypothetically, a length of say...RG-58 in odd multiples of a  1/8-wave 
make a great dummy load on paper with exceptionally high  return loss when 
the termination is either a short or open.  However,  I'm not sure how 
various transmitters would take to a Z of 50 ohms where  the reactance 
component radically changes between an open or shorted  termination but the 
power dissipating resistance portion is only a few  ohms.

Paul, W9AC  


TowerTalk  mailing  list


TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>