[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Overtones and velocity factors

Original poster: Paul Nicholson <paul@xxxxxxxxxxxxxxxxxxx>

Bart wrote:
> threw down a 4 foot ground plane, and hooked up the equipment.
> Found f1 at 598 kHz and decided I would increase f just to see
> if f2 was in range (nope, it's not).

Thanks Bart - even that one measurement is well worthwhile.

Tssp returns 595.3 kHz for the 1/4 wave, so that's a good sign.
Next higher mode is 1577 kHz.    I ran the model up to 23 quarter
waves (up at around 7574 kHz) but I don't think there's much
accuracy above about the 11/4 wave.

> There's not much use in doing much here if only f1 is attainable.
> Guess I really do need to upgrade my equipment (a new scope and
> generator are at the top of the list).

Well, the gear you've got is more than adequate for coiling. Surely
not worth investing in new kit just for one or two experiments.

Anyway I should leave it for now - the coil will always be there
for when an opportunity to measure arises - so long as you don't
destroy it with that silly sparking :)

Dest wrote (in another thread):
> i know nothing about this [velocity h/d] dependence - i`m reading
> archives currently, not your web site

There's nothing on tssp site about this - yet.  There's lots of
stuff that still needs finishing, checking, writing up (the whole
site could do with a revamp). Most of it has been talked about on
here.  The relevant part is that for low modes the overall wire
velocity factor is a smooth function of h/d ratio,


and the functions fitted to the measured and modelled points are

 1/4 wave:  Ph1(h/d) = ln(h/d) * 0.39 + 1.19
 3/4 wave:  Ph3(h/d) = ln(h/d + 2.7) * 0.6 + 0.21
 5/4 wave:  Ph5(h/d) = ln(h/d + 5) * 0.65 - 0.28

I've seen a reference to similar curves in radio handbooks (for the
fundamental only), but I think ours may be a little more accurate.

The measurement point that Bart has just supplied extends the
1/4 wave curve out to h/d = 32: curve predicts owvf = 2.54;
measured = 2.58;  error = -1.6%.

This smooth relation between h/d and wire velocity was first
pointed out to the list by one Ed Harris,


Recently it was rediscovered for us independently by Ed Phillips.
Paul Nicholson
Manchester, UK.