low band vhf antenna

[derick007]

I am looking for an antenna to transmit in the range 30MHz to 60 MHz, inside the house.

It is a very low power application and as I want to use inside the house I would like it to be as small as possible and of course as cheap as possible.

Suggestions would be welcome.

 

[JimB]

30 to 60 MHz, ????

Small and cheap equates to a gash piece of wire just trailing around.

What are you actually trying to do?

JimB

 

[RadioRon]

For very low power applications like yours, it doesn't make sense to use a full size antenna such as the classic quarter wave or half wave types. These are simply way too big. You need to use a structure that lies halfway between a classic antenna and a pcb tank circuit and what I am thinking of is simply a leaky coil. A wire coil that resonates (or is resonated with parallel capacitance, ie. a tank circuit) will also radiate and if it is a larger size than would normally be used on a pcb, with widely spread windings, perhaps this would be a suitable antenna. Its very much like a smaller version of a helical-loaded monopole or dipole. How big can it be? Several inches of length would be sufficient, I think.

 

[cannonball]

If your are talking about low power of 1 to 5 watts you can get a portable radio antenna.

 

[derick007]

I have a signal genny which can generate FSK which I intend to plug an antenna into if I can find one.

I also have a copy of MATLAB/SIMULINK with an SDR which I hope to use to view the spectrum of the FSK.

I would think the power would be in the order of a few hundred milliwatts at the most.

 

[Nigel Goodwin]

I have a signal genny which can generate FSK which I intend to plug an antenna into if I can find one.

I also have a copy of MATLAB/SIMULINK with an SDR which I hope to use to view the spectrum of the FSK.

I would think the power would be in the order of a few hundred milliwatts at the most.

Presumably you are aware it's illegal, and could cause interference for hundreds of miles?.

 

[JimB]

Why do you think you need to radiate a signal all over the lower VHF spectrum?
I don't think that you do.

You say that you have a signal generator (what make/model?) and an SDR (receiver).
What is wrong with setting the output of the signal generator to a suitable low level, say about 100uV (-70dBm) and making a direct connection with a coax cable from the output of the generator to the antenna connector of the receiver?

On a real world practical level, 100mW at VHF can go a LONG way, it will not magically stop inside your house, unless you live in a screened room of course!

JimB

 

[derick007]

I didn't think at such a low level, the signal would have travelled that far or cause interference.

Are there bands where it is legal to transmit test signals ?

I am sure I can make a direct connection between the signal genny and the SDR.

 

[unclejed613]

[have a look here] for portions of the spectrum where you can experiment with. the only one that seems to match your idea is 40.66-40.7Mhz. there's also the 46-49Mhz band used for cordless phones and toy walkie-talkies. in the 46-49Mhz band you are limited to 100mW (and i think the same is true for 40.66-40.7)

 

[Nigel Goodwin]

I didn't think at such a low level, the signal would have travelled that far or cause interference.

Such powers and frequencies have been used to transmit across the Atlantic, it goes a LOT further than you might imagine.

Going back MANY years, back when I was TV Engineer, a large part of the town lost one UHF TV channel (about a square mile or so), we reported it to the Interference service - and they located the problem. It was a masthead preamplifier that was unstable and oscillating - and these are only supplied with a couple of milliamps.

 

[derick007]

thanks for help and advice, looks like the ISM bands might be the solution

 

[Nigel Goodwin]

What exactly are you trying to do?, often when people are asking 'strange' questions it's because they are trying to do something in an unusual way.

 

[derick007]

I have a signal genny which can generate FSK which I intend to plug an antenna into if I can find one.

I also have a copy of MATLAB/SIMULINK with an SDR which I hope to use to view the spectrum of the FSK.

I would think the power transmitted would be in the order of a few 10's of mW in the ISM band.

 

[JimB]

I have already asked you "What type of signal generator?"
You have not replied.
You seem determined to connect this generator to an antenna.
I have told you that the easy way is to connect the generator directly to the antenna connection of the receiver
You have ignored that.

If you want our help, please give some specific details.
Your approach to the problem seems at best misguided.

JimB

 

[RadioRon]

You don't need to look far for an antenna. You needn't worry about a proper antenna with correct impedance matching if this is just an experiment. Just take a piece of hookup wire of any length between 6 inches and 36 inches, strip a bit of insulation off the end and arrange for that end to touch the center conductor of the signal generator's output connector. This will give you plenty of radiation for simple experiments. The only challenge may be to keep the wire from falling out of the connector.

 

[derick007]

Although it is only testing/experimenting, I would like to have a fairly accurate idea of the power I am transmitting. I know I can get this from a matched antenna, not sure about a variable length of conductor.

 

[JimB]

I would like to have a fairly accurate idea of the power I am transmitting.

Then you do not need an antenna.
You need a dummy load with a suitable impedance, power and frequency rating for the "transmitter", and an RF power meter.
Again the power meter should have a suitable impedance, power and frequency rating.

JimB

 

[RadioRon]

Although it is only testing/experimenting, I would like to have a fairly accurate idea of the power I am transmitting. I know I can get this from a matched antenna, not sure about a variable length of conductor.

OK. In this case, you can fabricate a simple quarter wave monopole out of some wire, and a length of coax to attach to your signal generator (with appropriate connector). If you decide to go this way, we can give you some dimensions, which will depend on frequency.