Hey Jim,
Thanks for taking the time to think about the question and even model it.
Really appreciate it.
You have a point that considering the extra time needed to make a second stub
is independent of the length of the stub, a better solution is to just make
another 1/4 stub.
Jim, one thing I find very confusing and contradictory, both from you and
George, is the type of coax. My first thought was to use LMR400 as George also
mentioned it in his book. Then you wrote that coax with foam dielectric is not
a good choice, but yet the Wireman #117 is really RG8X which in turn is same as
LMR240 which of course is also foam.
So, what coax should I use? I have to buy it anyway so I don't have any
preference.
Rudy N2WQ
Sent using a tiny keyboard. Please excuse brevity, typos, or inappropriate
autocorrect.
> On Jul 27, 2016, at 7:57 PM, Jim Brown <k9yc@audiosystemsgroup.com> wrote:
>
> 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?
>
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