Rev Den
NAXJA Forum User
- Location
- Plainfield, IL
splitz said:Oh, and Rev Dan, it may only be 4" but I want to work it as best as I can
Who?
Rev
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Coax length for single antenna setups
- Thread starter splitz
- Start date
Who?
Rev
You take the frequency (approx. 27-28 MHz) and get its wavelength (~36 ft.). Divide that in half for an ideal 1/2 wavelength=18ft. THEN multiply the half wavelength by the velocity factor of the cable you're using (it should be printed on the cable).
For RG-58 the vf is .66. 18*.66=11.88 ft.
For RG-8X the vf is .75. 18*.75=13.5 ft.
For RG-58 the vf is .66. 18*.66=11.88 ft.
For RG-8X the vf is .75. 18*.75=13.5 ft.
old_man
NAXJA Forum User
- Location
- Loveland, Colorado
OK, I'm an electrical engineer and have been designing radars and electronics for 40 years.
Given a properly operating and designed radio and a properly designed and installed antenna, the length of the coax does not matter, only it's impedance (aka Z). Z does not vary with length. Given the previous asumptions, coiling the coax has no effect. The problem comes when the radio is not a true 50 ohm source or the antenna is not set up to be a 50 ohm load. If both ends are a good 50 ohms and the coax is correct, 100% of the power should be radiated out the antenna. This is equivelent to 1:1 SWR and is ideal. A high SWR means that most likely the antenna is not operating at 50 ohms and needs to be adjusted. The Z of the antenna changes based upon it surroundings, whether it is mounted on a flat surface, a curved surface, a rack, or if it has other metal near. That is why antenna's are adjustable. Don't worry about the length, just adjust the SWR to the lowest possible number. If the numbers are crap, start looking at how the transmitter is grounded and how the antenna is mounted and grounded.
In theory, the shortest coax yields the lowest loss, but at 27mhz, it is a small difference. The reason why the 18' number comes into play has to do with wavelength theory and it can help if the load is badly mismatched, ie not 50 ohms.
Given a properly operating and designed radio and a properly designed and installed antenna, the length of the coax does not matter, only it's impedance (aka Z). Z does not vary with length. Given the previous asumptions, coiling the coax has no effect. The problem comes when the radio is not a true 50 ohm source or the antenna is not set up to be a 50 ohm load. If both ends are a good 50 ohms and the coax is correct, 100% of the power should be radiated out the antenna. This is equivelent to 1:1 SWR and is ideal. A high SWR means that most likely the antenna is not operating at 50 ohms and needs to be adjusted. The Z of the antenna changes based upon it surroundings, whether it is mounted on a flat surface, a curved surface, a rack, or if it has other metal near. That is why antenna's are adjustable. Don't worry about the length, just adjust the SWR to the lowest possible number. If the numbers are crap, start looking at how the transmitter is grounded and how the antenna is mounted and grounded.
In theory, the shortest coax yields the lowest loss, but at 27mhz, it is a small difference. The reason why the 18' number comes into play has to do with wavelength theory and it can help if the load is badly mismatched, ie not 50 ohms.
Ok, well I bought into the 18' x VF. My run was only 7 feet or so, but I don't know if it really makes a difference. I guess the only way to really find out is to tune it, change it, tune it again and see if the performance changes (which I'm not about to do).
But this brings me to a second question... If I'm not using a terminator on the antenna end, should I ground the braided wire to the chassis?
Oh, and if you're wondering why I'm not using a terminator, its cuz I'm stealing Sting25's mounting idea
But this brings me to a second question... If I'm not using a terminator on the antenna end, should I ground the braided wire to the chassis?
Oh, and if you're wondering why I'm not using a terminator, its cuz I'm stealing Sting25's mounting idea
