[CQ-Contest] Coax Stubs for SO2R

[Jim Brown]

Hi Rudy,

I remember seeing this years ago when I read George's book, so I went 
back to look at it to respond to your question. I have the second 
edition, which shows this concept in a table on page 36. I "built" a 
model in SimSmith for his Ref #4, which is for a pass frequency of 7 MHz 
and a null frequency of 14 MHz, using Belden 8237 (a good RG8). The 
stubs in the table are 11.68 ft, which makes them a quarter wave at 14 
MHz, so an eighth wave at 7 MHz. Tweaking the length of the open stub 
affects attenuation of the second harmonic, tweaking the length of the 
shorted stub does nothing. Removing the shorted stub has no effect on 
the second harmonic.

As I analyze this, the function of the 1/8-wave shorted stub is to 
reduce the mismatch on 40M introduced by the 1/8-wave open stub. George 
has a more complex explanation which I don't understand.

The open stub that's 1/8 wave on 7 MHz, is 1/4 wave on 20M, and because 
it's half the length of the 1/4 wave 7MHz shorted stub, it provides 5 dB 
greater attenuation than the 1/4 wave 7 MHz stub.

This double stub arrangement is just as sensitive to position along the 
line as a conventional shorted stub that's a quarter wave stub at the 
fundamental. But if you put it in the right place with respect to both 
antenna and transmitter, it will give you 5 dB better attenuation than 
the single longer stub.

BUT -- if you're willing to do two of the longer shorted stubs (quarter 
wave at the fundamental) separated by one-eighth wave at the fundmental, 
and properly place them, you'll get 20 dB more than one pair of double 
one-eighth wave stubs.

Hope this helps.

73, Jim K9YC

  On Wed,7/27/2016 9:59 AM, Rudy Bakalov wrote:
> Looking at W2VJN's book, page 28 caught my attention. Specifically, the use of type 3 stubs, where we use two 1/8th wave stubs in parallel instead of a single 1/4. Such stubs have higher attenuation at the expense of twice the loss (0.08 db vs 0.15 db).
>
> Other than being twice the work, is there a downside to using 2 X 1/8 instead of 1 X 1/4 stubs?