Greetings!

I picked up 2 Alford loop antennas from a ham fest. The antenna(s)
is for 112 mhz The data plate states VOR antenna 50 ohms. The
antenna(s) has Three N connectors one is to feed the RF from the
transmitter the other two are connected with a 31.5 inch length of
RG-214 coax. ( the 31.5 inch coax came with the loops)

I looked in the ARRL antenna hand book and it looks like the loop of
coax is configured as a 4 to 1 step up Balun??? The ARRL hand
book states that the Coax loop should be a electrical half wave of
coax : The coax loop is about 5 inches short of 1/2 wave at 112 mhz

When I hook up my MFJ-269 I get the following:

Freq: 112 mhz

RS - 9

XS - 68

Impedance : 95 ohms

VSWR 14.8 ????????

both loops have about the same readings!

WHY??? They are stamped with 50 ohms ?? but they don't measure out
to the data plate??
Can the length of the Balun loop have effect on the VSWR of the loops

What do the MFJ-269 readings tell you

Thanks!!!!!
Dave

I can only offer a couple of guesses.

Possibly the antenna is intended for receiving rather than transmitting.
If that's the case, then the mismatch isn't as serious as for
transmitting, although it would cause about a 6.5 dB signal loss
compared to a matched antenna if connected directly to a 50 ohm receiver.

The impedances and SWR you quote aren't quite consistent. 9 + j68 ohms
has an impedance magnitude of about 69 ohms, not 95. And it would result
in an SWR of 15.95, not 14.8, on a 50 ohm transmission line. The MFJ
calculates these from each other, so I have no explanation why you'd see
contradictory values like that.

I can't comment on the balun setup except to say that the length of the
transmission line loop is likely to have a significant effect on the
antenna impedance.

Roy Lewallen, W7EL

wrote:
Greetings!

I picked up 2 Alford loop antennas from a ham fest. The antenna(s)
is for 112 mhz The data plate states VOR antenna 50 ohms. The
antenna(s) has Three N connectors one is to feed the RF from the
transmitter the other two are connected with a 31.5 inch length of
RG-214 coax. ( the 31.5 inch coax came with the loops)

I looked in the ARRL antenna hand book and it looks like the loop of
coax is configured as a 4 to 1 step up Balun??? The ARRL hand
book states that the Coax loop should be a electrical half wave of
coax : The coax loop is about 5 inches short of 1/2 wave at 112 mhz

When I hook up my MFJ-269 I get the following:

Freq: 112 mhz

RS - 9

XS - 68

Impedance : 95 ohms

VSWR 14.8 ????????

both loops have about the same readings!

WHY??? They are stamped with 50 ohms ?? but they don't measure out
to the data plate??
Can the length of the Balun loop have effect on the VSWR of the loops

What do the MFJ-269 readings tell you

Thanks!!!!!
Dave