I have a question regarding dipole length, SWR and Smith charts.

I'm using a 40/80 trap dipole and wish to lengthen the 80M portion to
lower the SWR in the CW part of the band. I'm reading a SWR about
1.2:1 at 3.8MHz, 4.8:1 at 3.5MHz, which I wish to drop a little to
ease up on the auto-tuner in my radio. I plan on adding a couple of
feet and remeasure and I'll probably be close enough for rock and
roll.

Now, I use this antenna on other bands - I can load it on 20 (SWR
3), 15 (SWR 3) and 10 (SWR 3.2). I'd like to know what the impact
of adding another 2 ft to each side will do on these bands.

So - looking at the Smith chart, I can draw my circles for each band
and I can measure the physical length of each side of the dipole. How
do I relate length to SWR on the chart? And, would this be useful to
first-order? The whole function of a dipole is to radiate, so is this
a very lossy system, or would this be close enough to lossless to get
a good look?

Thanks for your thoughts

Mark/n0lf

Thanks for your reply, Roy

Rgds - Mark Sheffield/n0lf

On Sun, 12 Feb 2006 19:02:24 -0800, Roy Lewallen
wrote:

Mark Sheffield wrote:

I have a question regarding dipole length, SWR and Smith charts.

I'm using a 40/80 trap dipole and wish to lengthen the 80M portion to
lower the SWR in the CW part of the band. I'm reading a SWR about
1.2:1 at 3.8MHz, 4.8:1 at 3.5MHz, which I wish to drop a little to
ease up on the auto-tuner in my radio. I plan on adding a couple of
feet and remeasure and I'll probably be close enough for rock and
roll.

Now, I use this antenna on other bands - I can load it on 20 (SWR
3), 15 (SWR 3) and 10 (SWR 3.2). I'd like to know what the impact
of adding another 2 ft to each side will do on these bands.

It's impossible to say without knowing the L and C values making up the
traps. You'll just have to try it.

So - looking at the Smith chart, I can draw my circles for each band
and I can measure the physical length of each side of the dipole. How
do I relate length to SWR on the chart?

The relationship isn't a simple one. The SWR depends on the impedance,
and the way in which the impedance changes with length and frequency is
complex. Various approximations can be used, each with its own
limitations on accuracy and range of validity. The only practical way to
find the input impedance of all but the very simplest antennas is with a
computer modeling program. Traps add another level of difficulty. If you
know the impedance of the traps at a given frequency, you can model the
antenna at that frequency. But to know the antenna impedance over a
range of frequencies, you need to know what's in the trap, or you need
to know the trap's impedance at each frequency. If you don't know what's
in the trap, neither does the program.

And, would this be useful to
first-order? The whole function of a dipole is to radiate, so is this
a very lossy system, or would this be close enough to lossless to get
a good look?

I suggest trying a modeling program. The free EZNEC demo program from
http://eznec.com is adequate for modeling simple antennas. Again,
though, you'll need to know more about the traps to make even a guess at
what the antenna impedance will be, whether you use a program or some
approximate method.

Roy Lewallen, W7EL