Super Scanner coax

[Captain Kilowatt]

An open ended coaxial stub can be made to appear capacitive or inductive depending on it's length. If it is capacitive it will raise the resonant frequency of the element it is connected too and if inductive it will lower the resonant frequency. The coaxial stubs on the Superscanner tune the unused elements to a lower frequency hence they become reflectors. This is why the length is critical. Without the proper lengths the elements will not be tuned properly and could even become directors. Somewhere I have a n image of an antenna system that basically operates the same way and is what I plan to do on 80m. It consists of multiple half slopers arranged around a tower with switched feedlins. The inactive feedlines function as a stub turning all the unused elements into reflectors and giving some gain but more importantly some nulls to reduce QRM.

 

[jazzsinger]

An open ended coaxial stub can be made to appear capacitive or inductive depending on it's length. If it is capacitive it will raise the resonant frequency of the element it is connected too and if inductive it will lower the resonant frequency. The coaxial stubs on the Superscanner tune the unused elements to a lower frequency hence they become reflectors. This is why the length is critical. Without the proper lengths the elements will not be tuned properly and could even become directors. Somewhere I have a n image of an antenna system that basically operates the same way and is what I plan to do on 80m. It consists of multiple half slopers arranged around a tower with switched feedlins. The inactive feedlines function as a stub turning all the unused elements into reflectors and giving some gain but more importantly some nulls to reduce QRM.

I understand that Garth, its the shorted stub that has me curious, obviously 3 elements paralleled that are nominally 72 ohms would be about 24 ohm total if truly resistive which they won't be and spaced far enough apart which again they ain't ideal, so the other 26 ohm must be made up from the shorted 3/8 wave stub. i'm curious to see what the complex impedance of that is at unshorted end.

No doubt there's a lot going on in this antenna people don't understand.

from wrong length feed stubs which can be open circuit to increase element electrical size, which means they are inductive, if they were capacitive like you say would appear as directors

then we have people who don't grasp velocity factor assuming a 9 ft shorted stub is a 1/4 wave Q section when its actually 3/8 wave, which i believe supplies the impedance necessary to bring those 3 elements whatever their total impedance is to resonance in omni mode. There is very little info on anything other than 1/4 or 1/2 wave stubs. but that extra 1/8 wave must do something.

i had a play about on some impedance calculators and best match i could get assuming those 3 elements are 24 ohms was approx 60 ohms, or 1.2:1 swr, can't remember what value i gave unknown stub and obviously without one here and an analyser/vna can't be exact on reactance values.

i can get 51.3 ohm match on my calculator on phone, if that stub presented around 110 ohms in parallel with suspected 24 ohm total of 3 parallel elements, but its only guesswork. sq rt of 24 x 110 = 51.3, would be ideal for omni and i have a feeling spacing of elements may bring swr into line on beam mode. it's well documented if you add an element close to driven you can get 50 ohm feed. Two might be perfect.

whoever thought this antenna up was a smart mutha, no doubt the same guy that designed my beloved mighty magnum 3 as it was a gem too.

if anyone could analyse a 9ft shorted bit of rg58 at 27.185 mhz I think we may see an impedance reading in that vicinity.

 

[jazzsinger]

10 Meter "Super Scanner" Antenna Project - How to Modify the Original Super Scanner for 10 Meters!

Gives you the formula for the phasing lines.

http://www.cbtricks.com/ant_manuals/ant_specialists/ms_119/graphics/super_scanner_ms119_om.pdf

Original manual does not give you the length of the phasing harness.

If you are replacing the old coax then you just need to measure the length of the phasing harness you are replacing??

300 / 27.185 = 11.03m x 3.28 = 36.19 feet (freespace wavelength of 27.185 Mhz) /8 = 4.52 feet x 3 =13.57 feet x .66 = 8.958 feet x 12 = 107.5034" or a baw hair short of 9 ft for 3/8 wave stub.

just slightly under 1/2 an inch out. Probably insignificant on grand scale of things.

unlike your posted formula, it shows how they tuned it for ch20 and not ch1 like formula states.

Replace velocity factor for any other cable velocity factor, you should get similar result. That's why i don't rely on online calculators.

 

[Captain Kilowatt]

I understand that Garth, its the shorted stub that has me curious, obviously 3 elements paralleled that are nominally 72 ohms would be about 24 ohm total if truly resistive which they won't be and spaced far enough apart which again they ain't ideal, so the other 26 ohm must be made up from the shorted 3/8 wave stub. i'm curious to see what the complex impedance of that is at unshorted end.

No doubt there's a lot going on in this antenna people don't understand.

from wrong length feed stubs which can be open circuit to increase element electrical size, which means they are inductive, if they were capacitive like you say would appear as directors

then we have people who don't grasp velocity factor assuming a 9 ft shorted stub is a 1/4 wave Q section when its actually 3/8 wave, which i believe supplies the impedance necessary to bring those 3 elements whatever their total impedance is to resonance in omni mode. There is very little info on anything other than 1/4 or 1/2 wave stubs. but that extra 1/8 wave must do something.

i had a play about on some impedance calculators and best match i could get assuming those 3 elements are 24 ohms was approx 60 ohms, or 1.2:1 swr, can't remember what value i gave unknown stub and obviously without one here and an analyser/vna can't be exact on reactance values.

i can get 51.3 ohm match on my calculator on phone, if that stub presented around 110 ohms in parallel with suspected 24 ohm total of 3 parallel elements, but its only guesswork. sq rt of 24 x 110 = 51.3, would be ideal for omni and i have a feeling spacing of elements may bring swr into line on beam mode. it's well documented if you add an element close to driven you can get 50 ohm feed. Two might be perfect.

whoever thought this antenna up was a smart mutha, no doubt the same guy that designed my beloved mighty magnum 3 as it was a gem too.

if anyone could analyse a 9ft shorted bit of rg58 at 27.185 mhz I think we may see an impedance reading in that vicinity.

George, I mis-spoke above. (spoke? Typed? whatever) I did say an open stub however a shorted stub can do the same thing. I havent done the numbers but I suspect that using a shorted stub results in a line length significantly shorter than if an open stub is used. just guessing that if a 3/8 length works with a shorted stub it would require a 7/8 length to do the same thing with an open stub. Why waste cable. Use a shorted stub.

 

[Birddog2009]

Gee why don't you guys make it more complicated then it needs to be? SMDH.

 

[Marconi]

Rg 58U and i got 9' measuring old coax..

Birddog, don't get me wrong here, we're just trying to help, but seeing as you have the antenna why didn't you just measure the coax lengths that you asked about running to the relay box?

Is it possible that you just measured the 9' foot coax what is used as a shorted stub for the omni directional feature?

I agree with Jazz...we can't be sure about the lengths for the three coaxial wires going from the relay to the elements...because the manual I have shows these coax wires to be 29' feet long, and we all know that is not correct, and like Jazz notes...that is a mistake.

What is the length of three part D's that support this radiators for this antenna? See part #5 below.

 

[Captain Kilowatt]

Gee why don't you guys make it more complicated then it needs to be? SMDH.

There are some that are happy not knowing and then there are some that have a desire to know something by learning how things work and why.


Superscanner. It works. Go buy one. Skip this thread. Feel better?

 

[jazzsinger]

George, I mis-spoke above. (spoke? Typed? whatever) I did say an open stub however a shorted stub can do the same thing. I havent done the numbers but I suspect that using a shorted stub results in a line length significantly shorter than if an open stub is used. just guessing that if a 3/8 length works with a shorted stub it would require a 7/8 length to do the same thing with an open stub. Why waste cable. Use a shorted stub.

Makes sense Garth, even though in those days rg58 was so cheap in the states it would have amounted to a few cents per antenna, but its always the cb manufacturer way to cut every corner possible. I noticed your error but felt it unnecessary to dig up a man of your experience on it as knew you would realise it soon enough, I ain't petty and we have all mis typed, everything else you've said is sound, so I ain't going to hassle you over an obvious typo.

Eddie if you'd read my post and the link to the 10m mod it tells you the spreaders, part D are 35" originally, so its impossible for the rg58 that droops and runs through them to element to be 29", so the 2 was obviously the misprint and not the ' symbol

Plus BIRDDOG had already mentioned he measured them and were 3/8 wave just like the stub.

 

[Marconi]

Makes sense Garth, even though in those days rg58 was so cheap in the states it would have amounted to a few cents per antenna, but its always the cb manufacturer way to cut every corner possible. I noticed your error but felt it unnecessary to dig up a man of your experience on it as knew you would realise it soon enough, I ain't petty and we have all mis typed, everything else you've said is sound, so I ain't going to hassle you over an obvious typo.

Eddie if you'd read my post and the link to the 10m mod it tells you the spreaders, part D are 35" originally, so its impossible for the rg58 that droops and runs through them to element to be 29", so the 2 was obviously the misprint and not the ' symbol

Plus BIRDDOG had already mentioned he measured them and were 3/8 wave just like the stub.

Jazz, I agree with you...my manual does show what looks like 29' (feet). When I originally saw that notation in the manual I thought I saw 29" inches. I was looking for inches...and I just misread the notation.

I'm not sure I've seen where Birddog measured these wires as 3/8 wave though. However, I do recall him making a short post saying something was 9' long .

This is why I asked him if he might have measured the shorted stub instead of one of the 3 coax jumpers from the relay to the radiators. I also asked him, if he had the antenna, why he didn't just measure the dimensions he wanted.