[VHFcontesting] 70cm Antenna & Amplifier

[James Duffey]

Jim - For long distances on 432 MHz you will be relying on  
tropospheric scatter, often abbreviated troposcatter. For want of a  
lengthy description, troposcatter is scattering of our radio signal  
from roughly wavelength sized inhomogeneities in air density or water  
density in that part of the atmosphere below 30,000 ft or so. Some  
refer to the inhomegenities as blobs. That is probably an accurate  
description.

One of the classic articles on troposcatter is "Tropospheric Scatter  
Propagation", by G3YGF in November 1983 QST. If you are an ARRL  
member, you can download a PDF of this file from their web site.

You can calculate, at least to first order, how well your station will  
perform over particular paths rather simply.

I use the on line version of SPLAT!:

< http://splat.ecok.edu/ >

which will calculate path loss between two points. You need to know  
the coordinates of your station and the ones of the station you are  
trying to work, plus the antenna height. There is a database of  
stations on that web site which you can use and you can register your  
own location and that of stations you are trying to work.

SPLAT! is also available as a standalone application. SPLAT! is not  
the only software that will do this, RadioMobile will as well. THese  
are good programs, but if you use the standalone version, you will  
need to download the topographic information between you and the  
station you want to work. This data is available on the web, but can  
be tedious to download. The online version has all this online so that  
you don't need to download the topographic data.

Then you need to calculate the capability, or path loss capability of  
your station and the station you are trying to work. G3YGF shows how  
to do this. In brief:

The path loss capability, plc is given by:

plc = eirp + ers

where eirp is the effective radiated power of the transmitting station  
in decibels above 1 watt referred to an isotropic
radiating source. SO 10 Watts would be 10 dbW eirp, 20 Watts would be  
13 dBW, 100 wWtts would be 20 dBW and so on. and ers is the effective  
receiver sensitivity of the receiving station.


The effective radiated power is given by:

eirp = (10*log(P)) + Gtrans - transTL

where P is the transmnitter power in Watts, G is the transmitting  
antenna gain in dBi (decibels over
isotropic radiator), and TL is the transmitter stations transmission  
line loss in dB.

The effective receiver sensitivity is given by:

ers = -10*log(k*T*B) + Grecv - recvTL - threshold

the first term is the receiver noise floor where k is Boltzmann's  
constant,

k= 1.38*10^-23

T is the effective receiver noise temperature in degrees Kelvin  
(absolute temperature), which can be calculated from the
more coommonly available noise figure, nf, by:

T= ((10^(nf/10))-1)*290

and B is the receiver bandwidth in Hz.

Grecv is the receiving stations antenna gain in dBi

recvTL is the receiving stations transmission line loss in dB
and Threshold is the detector threshold for the mode being used. For  
CW and SSB it is 0, while for FM it is 10dB
depending on the modulation index.

Then you can calculate the expected signal to noise ratio over your  
path from:

snr = plc - pl

where pl is the path loss that you get from SPLAT! or Radiomobile.  
That path loss is a 50% number, half the time the path is worse than  
this, half the time it is better. The variation is usually +/- 7dB or  
so.

There are two path loss capabilities involved, one from from your  
station to the station you are trying to work and one from his station  
to yours. If you don't know what is at his end, assume that he is  
running a typical modest station on 432 MHz; 100 Watts out to 1 dB  
loss feed line, 2 dB noise figure and an antenna gain of 18 dBi up 35  
ft.

You can do excursions on this to see how much adding an amplifier will  
help. One thing to note is that going to CW will help a lot, due to  
the reduction in bandwidth. I have written all of this path loss  
capability in a MathPad application for easy calculation. It runs on a  
Mac, but if you want a copy, I can send it to you. MathPad itself is a  
free application. - Duffey

--
KK6MC
James Duffey
Cedar Crest NM