The entire thing is, we are making a mountain out of a molehill most of the time! Simply put the antenna ON correctly, following the mfr's instructions. Run whatever length of feed line you need to get to the radio and "fergit" about coax length (assuming you have a conventional metal body and sufficient ground. Next, lengthen or shorten the whip to
resonate the antenna to frequency. Like "Doc" said, the
impedance of the antenna system is important. This is the "50 ohms" we read so much about. Normally, at 27 MHZ, nothing else is really necessary to get a low SWR (and if you have a way to measure it, such as an antenna analyzer) & a 50 ohm match. If you haven't delved into antenna theory much, we'll leave it at that.
For those who just INSIST that the way to "tune" antenna is thru a
certain length of coax, I offer this:
Antenna theory is the SAME no matter what frequency you are operating on---whether it is 27 MHZ or
3 MHZ. That means, using the flawed and false doctrine of "coax length to tune the "swr'ssssssssss" (It is S W R, or VSWR-voltage standing wave ratio--and NOT SWR'SSSSSSS), we allegedly use 18 feet for 27 MHZ, right? So now if that is TRUE (it's NOT!), then I must use 120
FEET for that 3 MHZ, about 6
0 FEET for 7 MHZ,
32 FEET for 14 MHZ, and
16,7 FEET for 28 MHZ. Btw, if you use
468 divided by freq, in MHZ, it will
eliminate worrying about all that velocity factor stuff And so and so on. Using the CBer's "bible" about coax length, I must have a specific length of coax for EACH frequency I work, right? Let's muddy the water a bit!
In the amateur radio world, there is a mobile HF antenna called a "screwdriver", so named because it (formerly) used a hand-held cordless screwdriver converted to 12V to move a loading coil UP and DOWN within
tight fingers (look it up on the internet). By extending the loading coil UP, the frequency of operation goes DOWN. Retracting the coil DOWN into the mast INCREASES the operating frequency. What this means is, the operator can operate ANY frequency FLUIDLY and without interruption from 28 MHZ down to around 3.5 MHZ with an average SWR of 1.2 to 1.5
on ANY frequency he chooses. BUT WAIT!!!!!!!!!!!! I've gotta have a "certain" length of coax to work them frequencies or my "SWR" will be off, right? There's about 10 bands that hams work (and some have authorizations in other services that require other HF frequencies, etc.) so NOW I've gotta have a coax cut to 120 feet in my MOBILE:drool: for that 3 MHZ!!! AND ANOTHER one at 30 feet for 7 MHZ, and another one at 32 feet for 14 MHZ, YET
ANOTHER at 16 feet for 14 MHZ----
TEN OR MORE SPECIFIC PIECES OF COAX CUT TO A 'CERTAIN' LENGTH FOR EVERY FREQUENCY I NEED TO WORK!!! Where on EARTH am I going to put 10 or more coaxes in a MOBILE, ALL cut to a certain length:laugh:? Yet, hams and commercial operators operate thousands of frequencies with ONE mobile antenna, ONE coax cut to no particular length (whatever it takes to get from front to rear) and an SWR of 1,2 or less.
Another publication to look for is from Don Johnson, W6AAQ. It may still be available in disc (see "screwdriver antenna, DK3" and related): "40+5 Years of HF Mobileering" AND "What you Forgot to Ask about HF Mobileering". It will open your eyes!
73
CWM