Background
With a linkbudget it is possible to calculate if it is possible to obtain a communication-channel between two radio-stations.
To be able to make up this budget one has to know the output power, antenna gain and space attenuation. To make a more precise calculation one also has to know about noise-factors, cable attenuation and how much power the detector needs. I will not get into these more “accurate” calculations since this is made in several books and it would take to much time to write about.
The reason for having this part on my homepage is that having a strong neighbour-ham, as I have, one have to take into account that some day our antennas will point against each other and we will like to operate on the same band at the same time! This is the most “pessimistic” way of calculation, “normally” we both want to operate in the same direction and then the signal-strength will be slightly lower.
I also want to point out that this is only usable for direct communications. As soon as it is some kind of “conditions” we can’t calculate the received power level in this simple way. My receiver has been overloaded several times by stations in Britain or France when there are sporadic-E propagations or similar. If you would use the real distance under those conditions, these calculations will not give you any good or reliable result.
Mathematics
To be able to calculate the attenuation between the two antennas two things must be known, the used frequency and the distance between the antennas.
The approximate formula;
A(dB) = 32,4 + 20log(dkm) + 20log(fMHz)
At 144MHz we will get;
A(dB) = +75,6 + 20log(dkm)
Resulting “formula” for the 2m-band
If dkm is small, ie below 1 or 2 km, as it will normally be if you are calculating with “neighbour-hams” it can be neglected. The attenuation is therefore about 76dB.
If we neglect the attenuation in feeders, ie coaxial-cables to and from the antennas we will get;
PRX = PTX + GTX - 76 + GRX
Were GTX and GRX is the transmitter and receiver antenna gain in dB. The PTX is the output power in dBm. (To translate the output in W to dBm take a look here.)
With antenna-gain of 13dB at both the transmitter and receiver, ie a total antennagain of 26dB, we will get;
PRX = 0dBm with PTX = 100W
PRX = 10dBm with PTX = 1000W
Conclusion
These are only approximate figures, but it gives you a hint about the power levels your front-end and your pre-amplifier must be designed for!
If one has several neighbour-hams these figures will increase since the power levels are “additive”. If you have two hams, living in the same direction from your QTH, with high output power the increase will be about 3dB etc. One has also to consider all stations transmitting only in FM-mode!
In several countries there are pager-systems very close to the 2m-band, even FM-transmitters in the region of 88-108MHz can interfere if the filters are too wide!
If you live close to an airport and having a FT225, or another receiver with 10,7MHz or similar as the intermediate frequency, you could have problems with aircrafts passing by since these can transmit with quite high power levels and these signals could be transmitted on the image-frequency. It is therefore very important to filter the incoming signal.
All these signals will get into the first amplifier-stage in the front-end. This stage is often the one where distortion starts. If the signal is distorted nothing can get it back in its original format! Filtering and the use of amplifiers capable of handling large signals are therefore essential! One way of describing how much power an amplifier can handle is the interceptpoint.