On 1 jun, 22:50, "Antonio Vernucci" wrote:
Along several decades of radio hamming on the HF bands, I noted that the
measured SWR of all the antennas I have mounted (Yagis, dipoles) slightly varies
when the feedline length is changed by several meters. For 100W of forward
power, the reflected power could vary somewhat, e.g. from 2W to 5W or so,
measured on a Bird wattmeter. This behavior would seeem to deny the theory,
according to which *SWR is independent of feedline length (as long as the cable
attenuation remains constant).
Clearly the measured SWR change cannot be due to the change in the feedline
attenuation as, at HF, adding or cutting a few meters of cable would yield a
very small change in attenuation and hence a negligible impact on measured SWR.
Reading here and there, the most common theory explaining such phenomenon is
that, in presence of RF on the coaxial cable braid, the SWR meter reading is
influenced by the feedline length. I am not too convinced of that explanation,
also because I have invariably experienced the measured SWR variation phenomenon
with all antenna I have had, and I never had hot braid problems.
At that regard I got an idea that could explain the phenomenon, at least part of
it.
Reading coaxial cable data sheet, I noted that manufacturers typically give a
small tolerance on cable impedance (2 to 3 ohm). Let us then assume that the
feedline cable has a 53-ohm impedance, whilst the Bird wattmeter is 50 ohm
sharp.
If the 53-ohm cable is terminated on an e.g. 75-ohm (purely resistive) antenna,
the real SWR on the line would be 75/53=1.41 independently of feedline length
(if the attenuation variation with length is neglected). But the impedance seen
by the wattmeter obviously varies with the feedline length, and it can be easily
calculated that the seen impedance range results in an apparent SWR, on the
50-ohm wattneter, reading that varies from a maximum of 1.5 (when feedline
length is an even multiple of half wavelenght) down to a minimum of 1.33 (when
feedline length is an odd multiple of wavelenght quarters). For 100W of forward
power, the reflected power varies from about 4W down to about 2W.
Repeating the exercise with an e.g. 85-ohm load, the apparent SWR measured on
the 50-ohm wattmeter would vary from 1.7 down to 1.51 (reflected power varying
from 7W down to 4W).
You can get easily convinced that such variation is only due to the assumed
3-ohm difference in cable impedance.
With older cables having a nominal 52-ohm impedance, instead of 50, the
situation could get even more evident.
Any comment would be appreciated.
73
Tony I0JX
Hello Antonio,
In my opinion (when dealing with actual antennas) it can be:
1. your coaxial cable is part of the antenna (common mode current).
Changing the length, changes the common mode impedance. You can rule
this out by sliding some large ferrites along the cable close to the
VSWR meter, or change the grounding a bit and watch the difference (if
present).
2. your bridge inside the VSWR meter is not perfect. You can check
this by connecting known impedances (for example 56 Ohms resistor and
a 44.6 Ohms resistor and 100 ohms versus 25 Ohms).
3. The cables you are using are not exactly 50 Ohms. I think your
analysis is right. When you have cable with slightly different Z0,
readings depend on length. Of course when you extend with a good 50
ohms cable (directly connected to the meter), the reading should not
change. I did the math also and found also VSWR=1.7 and VSWR=1.51 for
85 ohms load connected to cable with Z0=53 Ohms. I didn't expect such
difference for just 3 ohms deviation from 50 Ohm.
4. Harmonics in the final amplifier (I hope that is not the reason).
Best regards,
Wim
PA3DJS
www.tetech.nl
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