[TowerTalk] Coax choice for crank-up - looking for TT experience/recommendations
jimlux at earthlink.net
Tue Jul 11 20:34:48 EDT 2017
On 7/11/17 2:51 PM, David Aslin G3WGN wrote:
> Hi Rich, Thanks for the thought. I developed an Excel spreadsheet
> for our 6Gs DXpeditions (TX6G, E6GG, ZL7G) that does the same as the
> arrg calculator but allows multiple segments each with selectable
> coax types to give a transceiver-to-antenna loss figure, so I can see
> the effect of cable selection on overall and segment loss;
If your segment lengths are not exactly a multiple of a half wavelength,
then using the "X dB/100 meter" number won't necessarily give you the
right loss, especially if the SWR is significantly different than 1:1.
That's because the loss (for HF) is primarily IR loss, not dielectric
loss. The current (and voltage) vary along the line. So if your VSWR
is 1.5:1, the loss (per unit length) at a current maximum is more than
twice the loss at a current minimum.
For multiples of a half wavelength, the usual equations for mismatch
loss work, because they assume an integrated loss over a whole cycle (or
half cycle - it's the same).
This is a way to get bitten when you're doing stuff like quarter
wavelength phasing lines with significant mismatch. The antenna may be
50 ohms, and you're using 70 ohm coax to get 100 ohms at the matching
point (two feedlines in parallel).
(actually, now that I think about it, the quarter wave is probably
another special case - the loss/length curve would be mirrored for a
half wave. So it's those 3/8 wave lines that cause issues, etc.
More information about the TowerTalk