18 feet of coax is NOT needed

Now we just need somebody to teach us all about shunt-matching.

 

This doesn't exactly look very scientific. It looks more like trial and error.
It would be nice to know if there was some sort of mathematical formula you could use to be right on the money the first time.

 

I just stumbled across this and hopefully it helps somebody.
From AC theory, two inductances in parallel will be less than the least inductance. Let's say you have two inductances in parallel, one is 2 uH and the other is 1 uH. This yields a total inductance of 0.67 uH.

L total parallel = (L1XL2)/(L1 + L2)

Two inductances in series sums the inductance of both coils. So two 1 uH inductors in series yields 2 uH. L total series = L1 + L2.

You'll need one of these LC meters to wind the inductor.

 

Ok so in the real world of antennas that looks like the Jed Clampett version of a antenna..... What's all that gaining you ????? HMMMMMMMMMMM---- ARE YA TALKING/RECEIVING 50 + MILES

 

Uhhhhhhhhh...........YES! Especially if you're talking about 40 Meters. Not just 50 miles, but worldwide!

Anyway back to coax length--which comes up from time to time. Generally speaking coax length IS a myth. Oh.............you can do that, but presents a false picture of what your antenna is actually doing SWR-wise. Shunt-matching at the feedpoint IS the correct way to match an HF antenna. I've posted it here before. Take that 40 meter antenna for example. Most CBers take the coax length myth as gospel. Mostly because its the only way they've been shown to do it. What if I could show you an all-band HF antenna that covers 80 thru 10 meters (and ll meters,as well) that has about a 7 foot coax feed and is feed point matched in the manner shown by the illustration above? IF that thing about coax length was true (and it mostly isn't), then the formula for determining the frequency of the antenna would be 468/frequency in MHZ/ 2. This is the same way a dipole is done because there's two parts of the dipole. The vehicle is half the antenna so we'd get 17.2 feet/2= 8.6 +++ a little bit. OR the classic 9'. Now. Let's go up to 14 MHZ. And 468/14=32+++/2=16 feet +++ Keep doing that until you get up to 75 meters. 468/3.855=121.5-ish/2=60.75 feet! Continuing, there is an amateur HF antenna called "The Screwdriver" that covers ALL those frequencies continuously thru out the HF spectrum. It is motorized and moves fluidly from 80-10-11 meters without any detents, stops, or other changes needed at the touch of a button. SWR, properly installed, will range from 1.6 or 1.2 from top to bottom. SO! IF the coax length thing was true, then I'd have to have about 10 coaxes, ranging from 8-9 feet to 60 feet to operate all those frequencies!! That is, IF coax length was "gospel". IT is NOT! My all-band screwdriver has a coax of about 7 feet, is feed point matched with a little coil , and has 1.2 SWR from 10, 11, 12, 20, 40, 75 Meters. Where would I PUT all that coax IF I had to have a specific length for each frequency I worked?????? It just ain't so!

GF

 

Gadfly do not use that dipole formula.

Rabbi said it is not correct.

 

I don't recall saying it wasn't correct but I do recall saying I use a metric formula which is slightly different without using Imperial measurements

 

Gadfly, you may have just helped the cascadia drivers solve there antenna woes. I had seen pictures of a couple of drivers using a screwdriver antenna on their trucks. Seemed like over kill at around $400. As you know MFJ has a similar antenna ( but not auto tune or physically as big), with a wire to tap into the coil. Once they had the antenna resonance for the 11 meter band, they would be all set. Then all they would have to worry about is keeping it from blowing off the truck while driving down the road.

 

I always thought a horizontal dipole would be the perfect DX antenna on the rear of the cab mounted up really high on that particular truck.