Wouldn't a reading of Z=50 equal SWR=1??

[cillcalley]

I have a Mini60 antenna analyzer (a copy of the Sark 100), and what I find strange is that sometimes when I get a reading of Z=50 ohms, the analyzer will give me a reading of SWR =>10. Is this analyzer broken, since Z=50 ohms should equate to SWR=1, or is it something obvious that I am missing??

Thanks!

-Bill

 

[JimB]

You must define the system impedance.

I am not familiar with the Mini60, but I have an AIM4170 where it is possible to define the impedance.

VSWR is with respect to the system impedance.
In a 50 Ohm system, a 50 Ohm load is 1:1 VSWR
In a 75 Ohm system a 50 Ohm load is a 1.5:1 VSWR

JimB

 

[JimB]

A bit of a pedantic rant...

SWR - Standing Wave Ratio, or to give it its full title VSWR - Voltage Standing Wave Ratio, is as the name implies a RATIO.

So to say that the SWR = 1 or the SWR =3 etc is technically wrong.
Those familiar with the concept should understand that what is really meant is an SWR of 1: or 3:1 or what have you.
Beginners or those not too familiar may not appreciate the significance.

To add to the potential confusion, my Antenna Analyser displays SWR as a simple number rather than a ratio.

RantMode = OFF

JimB

 

[cillcalley]

Hi JimB,

Thanks for the info! What I have is an RX antenna, fed by some crappy 75 ohm TV coax into a random length dipole (46 feet long), used to receive both the 20 and 40 meter CW bands. However, I find it odd that the antenna meter decides that a 50 ohm impedance is >10:1 VSWR (which was centered at around 11MHz after a frequency sweep from 6Mhz to 16Mhz)??

 

[JimB]

A half wave dipole which is 46 feet long will (should) show a resonance at about 10MHz.

In theory a half wave dipole should have a feed impedance of about 75 Ohms at resonance (some sources give the impedance as 72 Ohms).
There are many factors which will affect the feed impedance of a practical dipole, so what you will actually see in your case is open to conjecture.

At frequencies other than the resonant frequency, the feed impedance will be different, to complicate things the coax will have an impedance transforming effect.
So what impedance and hence SWR you will see at the end of a random length of coax is anybodies guess.

JimB