I want to use an existing terrestrial DTV antenna, similar to the one pictured, as a plug in antenna for use on a cell phone in remote areas.
The plan is to use a high quality 50-75 ohm matching transformer, DC-2500MHz.
Is this likely to be successful?
Cheers.

 

I don't know, it is hard to tell by just looking. The best approach would be to sweep the feedline, transformer, and antenna with a VNA. That way you will be sure. Broadband approaches generally don't work as well as more narrowly focused approaches IMHO.

 

I want to use an existing terrestrial DTV antenna, similar to the one pictured, as a plug in antenna for use on a cell phone in remote areas.
The plan is to use a high quality 50-75 ohm matching transformer, DC-2500MHz.
Is this likely to be successful?
Cheers.


View attachment 281685

What kind of input are you feeding it to. From the looks of the antenna it has a balanced output - two signals (+/-) and no ground , If you are feeding an an unbalanced input - one signal and one ground.l you will need the transformer as a balan - balanced to unbalanced. Do you know the input impedance of what you are feeding it to?
I think you could what you have, and you will probably get a "not bad" interface, although knowing the antenna output and device input impedances would help.
You might check the Amateur Radio Relay League, if they are still available.

 

Welcome.
Impedance is tied to a frequency. 75 ohm from DC to 2500MHz is pure fantasy. Any change in frequency changes the impedance, will never stay fixed to 75 ohm.

where each of the reactances


Both reactances above have the factor f. (frequency) Change f and Z changes.

I have never seen specifications of a television antenna plotting its impedance versus frequency. Pure fantasy.
Matching that antenna to a television tuner impedance is pure fantasy. Never seen a data sheet for a TV tuner plotting impedance versus frequency.

Using a 75 to 50 ohm transformer to match a cell phone falls in pure guess. Losses from coaxial from that antenna to the phone can destroy a lucky guess of improving the phone reception with it.
You do not know what is the frequency band your phone works at a given moment. Matching that to 50 ohms is pure guess.

But try, perhaps you are lucky.

 

What kind of input are you feeding it to. From the looks of the antenna it has a balanced output - two signals (+/-) and no ground , If you are feeding an an unbalanced input - one signal and one ground.l you will need the transformer as a balan - balanced to unbalanced. Do you know the input impedance of what you are feeding it to?
I think you could what you have, and you will probably get a "not bad" interface, although knowing the antenna output and device input impedances would help.
You might check the Amateur Radio Relay League, if they are still available.

The 75 ohm coax from the (ungrounded) antenna will go to the 75 ohm side of the matching transformer, and the 50 ohm output from the transformer will go via a short(ish) 50 ohm coax to my phone. Cell phone antennas are generally 50 ohms.

 

Welcome to AAC.

It might work if the antenna is resonant in the band the phone is using. That's easy to check before bothering to hook anything up. If the phone is poorly matched the output amplifier will either reduce its power to protect itself or get hot and eventually fail. In either case the output from the antenna will be very small.

I don't know where you are of what the allocations for DTV and Mobile look like, but I suspect there is little overlap.

 

Welcome.
Impedance is tied to a frequency. 75 ohm from DC to 2500MHz is pure fantasy. Any change in frequency changes the impedance, will never stay fixed to 75 ohm.

View attachment 281788where each of the reactances
View attachment 281790
View attachment 281791
Both reactances above have the factor f. (frequency) Change f and Z changes.

I have never seen specifications of a television antenna plotting its impedance versus frequency. Pure fantasy.
Matching that antenna to a television tuner impedance is pure fantasy. Never seen a data sheet for a TV tuner plotting impedance versus frequency.

Using a 75 to 50 ohm transformer to match a cell phone falls in pure guess. Losses from coaxial from that antenna to the phone can destroy a lucky guess of improving the phone reception with it.
You do not know what is the frequency band your phone works at a given moment. Matching that to 50 ohms is pure guess.

But try, perhaps you are lucky.

Cell phones use a wide range of frequencies, so any antenna, including the one inside every cell phone, is a huge compromise, but they work most of the time.
All I am thinking of doing is using a lot more signal gathering area, i.e. the TV antenna, which unfortunately is 75 ohm impedance. The transformer will provide a matching interface so I have a 50 ohm output to connect my phone to, which has an input impedance of 50 ohms.
The figures across the frequency range are shown in the pic.
The frequencies used in the areas I frequent are: 700MHz, 850MHz, 900MHz, 1800MHz, and 2100MHz.

 

Welcome to AAC.

It might work if the antenna is resonant in the band the phone is using. That's easy to check before bothering to hook anything up. If the phone is poorly matched the output amplifier will either reduce its power to protect itself or get hot and eventually fail. In either case the output from the antenna will be very small.

I don't know where you are of what the allocations for DTV and Mobile look like, but I suspect there is little overlap.

DTV uses 200MHz to 800MHz and cellphone uses 700, 850, 900, 1800, 2100MHz.
Doesn't look too promising :-(

 

Cell phones use a wide range of frequencies, so any antenna, including the one inside every cell phone, is a huge compromise, but they work most of the time.
All I am thinking of doing is using a lot more signal gathering area, i.e. the TV antenna, which unfortunately is 75 ohm impedance. The transformer will provide a matching interface so I have a 50 ohm output to connect my phone to, which has an input impedance of 50 ohms.
The figures across the frequency range are shown in the pic.
The frequencies used in the areas I frequent are: 700MHz, 850MHz, 900MHz, 1800MHz, and 2100MHz.

Multiband antennas are not a "compromise", they are designed to be resonant in the appropriate bands. There will always be resonant frequencies and, unlike the passive DTV devices, your phone is a transmitter. Reflected power in the case of a receiver means poor reception. A transmitter, on the other hand, is going to produce heat, and this can and does kill output amplifiers. If it doesn't, it is because the PA is designed to protect itself by reducing power.

So you have a relatively small signal coming from the phone, you have the loss is the coax which will be substantial if the run is any appreciable length, you have the inevitable mismatch between phone and antenna causing reflected power and its concomitant loss. What does make it out of the antenna is likely to be a very tiny signal.

You might want to get a very affordable NanoVNA vector network analyzer and characterize the antenna to see where is it actually resonant. It will plot exactly what you are getting from the antenna and is a nice bit of kit to have around.

 

DTV uses 200MHz to 800MHz and cellphone uses 700, 850, 900, 1800, 2100MHz.
Doesn't look too promising :-(

No, it doesn't.

 

... the TV antenna, which unfortunately is 75 ohm impedance. The transformer will provide a matching interface so I have a 50 ohm output to connect my phone ...

No, the TV antenna is not 75 ohm. That is a fairy tale. That impedance may coincide at one frequency, not all of them. Impedance is tied to a frequency, not to a band.

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That is unbeatable.

 

The 75 ohm coax from the (ungrounded) antenna will go to the 75 ohm side of the matching transformer, and the 50 ohm output from the transformer will go via a short(ish) 50 ohm coax to my phone. Cell phone antennas are generally 50 ohms.

So, you are connecting one side of the antenna to one of the 75 ohm transformer's input, and the other side of the antenna to the other 75 ohm transformer's input. One side of the 50 ohm output will go to the center conductor of the coax, and the other side of the 50 ohm output will go to the outer conductor of the coax which is grounded at the phone. It should work even though the impedance matches might not be optimum.