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?
_______________________________________________
CQ-Contest mailing list
CQ-Contest@contesting.com
http://lists.contesting.com/mailman/listinfo/cq-contest
|