At Where is the best place to measure feed point VSWR I discussed location of the VSWR meter and projection of its reading to another point on a known transmission line.
A correspondent has taken me to task and citing Telepost’s LP-100A manual:
Placement of LP-100 in the transmission line:
Where should I place the LP-100 in the transmission line between the rig and antenna?
The best place for the LP-100 coupler to be inserted is between the rig (including any amplifier) and the antenna tuner or antenna. The tuner should be considered part of the antenna system. Use of an internal tuner in the rig will result in inaccurate power and SWR readings on the LP-100 (or any other external wattmeter). The LP-100 is designed to work with a 50 ohm source impedance. When an internal antenna tuner is used, the output impedance of the rig will no longer be 50 ohms. You will also experience a power loss in the tuner of up to 20% or so, which will be seen on the LP-100. To measure an antenna’s actual impedance requires that any internal tuner be bypassed, as well as any external tuner which follows the LP-100. With an external tuner following the LP-100, you can adjust the tuner while monitoring SWR or Return Loss on the LP-100 until a match is found. Switching the external tuner between operate and bypass will show the effect of the tuner.
The inference that the Thevenin equivalent source impedance of all or most transmitters is 50Ω is wrong, though they may often be designed for a load of 50+j0Ω (with some stated tolerance)… there is a difference.
But more importantly, the statement
use of an internal tuner in the rig will result in inaccurate power and SWR readings on the LP-100 (or any other external wattmeter) is concerning. The VSWR (SWR) at any point on a line is dependent on the load impedance and transmission line characteristics for the line between the load and point of interest. It is independent of the source characteristics (Duffy 2008).
If for some reason the LP-100A does not correctly indicate VSWR because of some source characteristic, then the LP-100A is deficient.
Likewise, if the LP-100A does not correctly indicate forward and reflected power components at some point in a nominal 50Ω transmission line because of some source characteristic, then the LP-100A is deficient.
I would opine that these are just two of several errors in the LP-100A manual, errors that question the knowledge of the manual’s author.
- The VSWR (SWR) at any point of interest on a line is dependent on the load impedance and transmission line characteristics for the line between the load and point of interest. It is independent of the source characteristics.
- The Telepost LP-100A manual’s suggestion to the contrary is wrong.
- If in fact as claimed, the LP-100A depends on the Thevenin equivalent source impedance being 50+j0Ω, it is a deficiency of the instrument.
- Duffy, O. Apr 2008. Telegraphers equation. VK1OD.net (offline).
- Telepost. Jun 2012. LP-100A digital vector RF wattmeter. Telepost Inc (accessed 26/02/2015).
- Telepost. Sep 2013. LP-100A digital vector RF wattmeter. Telepost Inc (accessed 28/05/2015).