Hello,

I currently have a transmitter working similarly like morse code at 5MHz with a transmitting power of 50mW. Trying to design the receiver end, I need help designing an antenna, could I possibly just use a ferrite core inductor and variable capacitor with a long wire to tune to 5 MHz? Will this work if the distance I would like to transmit is about 10 to 20 meters?

Thanks in advance

 

Back of the envelop.

At 5 MHz. the wavelength is:

300 / 5 = 60 Meters.

This means that 10-20 meters away you are in the near field (usually 2λ). The RF gotta go somewhere so the receive antenna will hardly matter. The challenge will be keeping the RF out of your receiver front end. We are headed into a solar max and 50 mW might propagate a bit further than you would expect. If you interfere with anybody, you can expect a visit from the FCC. They take a really dim view of what you are doing. If it was 5 GHz. I would be less concerned with "unexpected" long distance propagation.

In real time, you can get a propagation picture from the following website:

http://www.vhfdx.info/spots/map.php?Lan=E&Frec=7&ML=M&Map=NA&DXC=N&HF=S&GL=N

 

Hello Papabravo

I wouldn't like to see the FFC at my door so how much power should I transmitting?

 

It is not a question of power. It is a question of frequency and are you interfering with the licensed user(s) of the frequency. On the HF frequency bands you can't be sure if your signal will go across the field or across the continent. There are frequency bands designated as ISM (Instrumentation, Scientific, and Medical) bands. Unlicensed operation on these bands may be more practical than at 5 MHz.

http://en.wikipedia.org/wiki/ISM_band

Here is the wiki on 47 CFR 15

http://en.wikipedia.org/wiki/Part_15_(FCC_rules)

You might find this amusing as well

http://www.tvwiki.tv/wiki/The_Master_List_of_Part_15_Radio_Stations_of_North_America