QRP operators often recommend this bridge and I also like to use it.
However, there is one problem. If during antenna tuning LED indicator dims, it still may be far from right. The bridge compares the impedance at the antenna connector with the real value 50R. Several times I have verified that the output impedance of various QRP equipment is quite different. Then, after switching the transmitter output to the antenna, we lose a significant part of the transmitted power.
At least it is good in the second step to tune the antenna maximum current (or voltage) even at the cost of a slight SWR resistive bridge unbalance.
Not if you use it as instructed, i.e. with a 50 ohm Tx
The bridge compares the impedance at the antenna connector with the real value 50R.
Several times I have verified that the output impedance of various QRP
equipment is quite different.
Indeed, which is why you must read the documentation before you use any piece of equipment. If you have a transmitter that has an output impedance that is not 50 ohms then you need an atu/swr meter that matches that value instead of a 50 ohm one.
Of course it is quite simple to modify the resistive SWR bridge so that it matches your transmitter. The difficult part is often measuring the output impedance of a homebrew Tx.
it is even a bit more complicated - impedance of the output stage of the transmitter varies with supply voltage (battery powered) - and can be different for each band and another rf power value setting.
I measured it using a directional coupler, differences related to real 50ohm are not negligible - even at a factory tcvr!
I think very simple and reliable is then also always good to find the maximum antenna current.
I measured it using a directional coupler, differences related to real
50ohm are not negligible - even at a factory tcvr!
This is very true and seldom appreciated. It becomes apparent if you use an antenna current meter, that very few transmitters have an output impedance of anything like 50 Ohms. Quite funny then that we spend so much time and (often wasted) effort in getting the VSWR low.
spend so much time and (often wasted) effort in getting the VSWR low.
Yes, but since most recently produced commercial transceivers reduce their output automatically when the SWR is high, this effort is still necessary.
The problem I found when I tried an LED SWR indicator is that in bright sunlight it is sometimes difficult to see what the LED is doing, particularly when there is snow on the ground. I have in my junkbox a couple of those little meters that they used to put in portable tape recorders (remember those?) and will probably include one next time I build a portable ATU.
I can recommend the “Stockton Wattmeter” type arrangement - can be left permanently in line, and made very compact using FT37-43 cores and RG174 for the through-lines.
You can often pick up small edgewise meters with an FSD of 200 microamps that are ideal, or Maplin do a “Signal Strength” meter that’s not so compact but would do. I prefer to use two meters rather than switching between “forward” and “reverse” power. Absolute calibration not necessary - just set the forward power to read full scale when rig producing maximum smoke
