Imax 2000 EXPOSED!

[nomadradio]

There is a velocity-factor issue with an antenna made from metal tubing.

A wire will tune at a slightly-longer measured length than than tubing at the same resonant frequency. The thicker the tubing, the shorter it will measure when tuned to resonate at the same frequency as a wire antenna.

The velocity factor used in most 'antenna handbook' calculations says "97%" for HF-band wire antennas. The thicker your radiating element, the smaller that number gets. The famous "234 divided by frequency in MHz" for a quarter-wave antenna includes that 97% VF.

73

 

[Needle Bender]

So we're do you measure from to tip for that fine line bringing it to 27.185 or 27.000 at .64

Well you really want to make sure that your .64 is a .64 at the highest frequency you plan to use - since any longer tends to lose the low TOA and sends the RF heading for the space shuttle, in that using a .64 above the tuned .64 frequency makes it effectively longer at those higher frequencies.

I measure from the top of the matching network in the base of the Imax behind the rings & ring threads to the tip, and remember, there's a several inch long capacitor about 134" down from the tip to consider on this oddball antenna which thinks it's a 1/2 wave.

There is a velocity-factor issue with an antenna made from metal tubing.

A wire will tune at a slightly-longer measured length than than tubing at the same resonant frequency. The thicker the tubing, the shorter it will measure when tuned to resonate at the same frequency as a wire antenna.

The velocity factor used in most 'antenna handbook' calculations says "97%" for HF-band wire antennas. The thicker your radiating element, the smaller that number gets. The famous "234 divided by frequency in MHz" for a quarter-wave antenna includes that 97% VF.

73

For aluminum tubing in air the velocity factor I've found listed is .9979.

- Often when you have a wire radiator with insulation, you can tend to get a lower VF due to the insulation material, though the 75M Delta Loop I designed for my neighbor calculated out to a perfect 11803/freq. using standard 12ga THHN with a polyvinyl jacket, so go figure - lol.
He's got a 1.09:1 reading at the target frequency on his Palstar digital meter using a 1:1 balun designs balun just outside the wall behind the radio and a 4:1 at the feed point, after cutting it to 258' and then having to cut 3' from each side to get it under 1.1:1 ending up right at 252' just as the 11803/freq formula shows.

Also, you have to consider the circumference of any tubing as adding electrical length causing a shorter resonant or tuned element length than one might calculate, but I don't believe it's part of the VF, rather more a function of the circumference measurement adding electrical length.

 

[Needle Bender]

Here's a cool graph I found which supports your post, but leaves us scratching our heads as to why the Delta Loop wants to calc out perfectly.

I figured about 7K:1 - 10K:1 when using about 1/16" wire like 14ga at a 434.25" wavelength.

 

[Paul-W5lz]

Don't get too tied up with those numbers, they will almost never come out exactly right. Everything around that antenna can affect it, so the exact length is variable to make the thing work in some specific situation. Tuning an antenna is what makes an antenna work well in the situation it's in. Get close then 'tweek it'... Or would that be 'twerk it'??