Determining TX frequency of cordless telephone

[Warren Post]

I´ve been given an old long range cordless telephone. The base station
takes two antennas (transmit and receive) but only came with the RX
antenna. I can make a homebrew TX antenna but need to know the
transmission frequency in order to do so.

Naturally the unit came with no manual, no specs, and no schematic. No
manufacturer´s name appears outside or inside the equipment, not even on
the PCBs. There are no FCC or UL numbers to look up. I do have a clue,
however: The base unit´s reception antenna is clearly labled ¨RX: 70
MHz¨. And inside the handheld unit there is a component stamped ¨70.365
H.ELE. CH23 1J¨. So 70.365 MHz seems to be the base unit´s RX frequency.
Also inside the handheld are two apparently identical commponents stamped
39.040 and 10.245 respectively. So perhaps one of these is the base
unit´s TX frequency in MHz?

To test my idea I looked inside the base unit. There are four apparently
identical components, stamped 59.665, 10.245, 49.740, and ¨10M 15A¨,
respectively. So is it safe to conclude that 10.245 MHz is the base
unit´s TX frequency and that I should make the antenna for that frequency?

 

[Jerry G.]

What you are most likely looking at in the handset, are more than
likely, the crystals for the local oscillators for the TX and RX. The
actual carrier frequency is a multiple of the crystal's fundemental
frequency.

A good way to know the actual frequency is to use a scanner to find it,
or for the best accuracy, use a spectrum analyser to see where it is.

If you go to any of the electronics parts suppliers that sell parts for
radio and TV servicing, they may have some whip antennas that will fit.
Most of these were very standard. If you cannot find one of the
quality, or proper matching, try any of the companies that sell
communications equipment.

--

These older type cordless phones usually worked in the 36 to 75 mHz
freqency band, depending on the country that it was to be sold in.

The newer 2.4 gHz phones are much more superior to what you have. The
new high end consumer phones are lighter, and can go farther.

Panasonic has a new line of them that is called the Gigarange. The
older Gigarange phones were at 2.5 gHz. The new ones are at 5.8 gHz.
This series can go very far.

An example of a few:
http://www.ahernstore.com/2linecordless.html

A friend of mine lives in the country. He has been about 1/2 mile from
his home with this 5.8 mHz phone, and it still works. With his
Gigarange, he can call out on his home phone, even if he is right
accross the lake.

I have take a 2.4 gHz up to several thousand feet from my home, and it
still worked.

Jerry G.
======

 

[Gary J. Tait]

I´ve been given an old long range cordless telephone. The base station
takes two antennas (transmit and receive) but only came with the RX
antenna. I can make a homebrew TX antenna but need to know the
transmission frequency in order to do so.

Naturally the unit came with no manual, no specs, and no schematic. No
manufacturer´s name appears outside or inside the equipment, not even on
the PCBs. There are no FCC or UL numbers to look up. I do have a clue,
however: The base unit´s reception antenna is clearly labled ¨RX: 70
MHz¨. And inside the handheld unit there is a component stamped ¨70.365
H.ELE. CH23 1J¨. So 70.365 MHz seems to be the base unit´s RX frequency.
Also inside the handheld are two apparently identical commponents stamped
39.040 and 10.245 respectively. So perhaps one of these is the base
unit´s TX frequency in MHz?

70.365Mhz is the RX LO, (LO-IF=RF), so 59.665Mhz is the handset RX
frequency.
1st IF is 10.7Mhz, 2nd IF is 455 Khz. and therefore 10.245Mhz is 2nd
IF LO.

The handset transmits at 39.04 Mhz.

To test my idea I looked inside the base unit. There are four apparently
identical components, stamped 59.665, 10.245, 49.740, and ¨10M 15A¨,
respectively. So is it safe to conclude that 10.245 MHz is the base
unit´s TX frequency and that I should make the antenna for that frequency?

The freqencies are opposite to the handset.

49.740 is the RX LO, receives 39.040 Mhz, TX is 59.665Mhz. 2nd IF is
the as the handset.