Article belongs to VU3KPL Kiran
I hold restricted grade Amateur Radio license with voice mode endorsement. It allows me to transmit only of 50W on HF. When I wanted to set my station at Bangalore, I wanted it to optimally use all the allowed power and get on the air. Referring to many internet information available on various subjects, i found that most of the hams(including me!) never optimize their station and take advantage of power. Although power shows up as 50W or 100W going out of their rig, how much is actually transmitted out their antenna in a desired direction is very important.
Lets look at some of the aspects which i considered (the way in which i understand them):
This metal structure everyone knows is very important for transmission/reception.There are couple of important aspects in the antennas 1. resonance at desired frequency 2. line impedance match 3. gain in desired direction
1. Resonance: when the RF wave finds matching length of conductor we call it as antenna is resonating at that frequency. Ideally only resonant frequency should be available to listen or transmit efficiently. However antenna tends to have behavior of acceptable bandwidths. That is the range of frequencies for which efficiency becomes acceptable. For small space antennas like magnetic loops, loaded antennas etc this range is very narrow. When length of conductor is longer(2-3 times) than wavelength of maximum desired frequency wavelength, it can resonate at various frequencies below it. So a tuner can make sure that conductor accepts or transmits desired frequency.
2. Line impedence (Z): This is collective measurement of transmission system i.e transceiver i/o, line and antenna feed point itself. Normally amateur radio rigs have 50 ohms line connectors and coaxes are 50 ohms rated. Antennas should also will have to have that 50 ohms rating. Else reflection of waves back to rig from antenna will occur which is called SWR (Standing Wave Ratio). Getting SWR to within 1:1.1 yields in best results.
3. Gain in desired direction (DBd gain in decibels measured with a dipole reference point) Do we require to transmit in a direction which is not intended to be?. Like a 50 W light bub hanging over head without its reflector, antenna would have no focus. Put a reflector all light now with same 50W bulb seems to be with more brightness for the person infront of it. Put a lens in-front of the bulb, light rays gets sharper and intense in-front of it. Similarly in RF, using a single element vs using two or more element has its advantages. All this gain comes with its mechanical and spacial overheads too. Work on a space optimized and gain optimized antenna at a clear height of around 30ft from ground or above.Check nearby obstacles like highrise buildings, mountains etc. these will have adverse impact on signals in their direction.
Chose best available cables for your needs. Onetime investment of few hundred Rupee more will take you through seven sees for years. Chose bare copper over tinned copper braids.
Typically Indian station antennas are just above the roof level and coax lengths are around 50ft.
Cable loss : refer to http://www.timesmicrowave.com/cgi-bin/calculate.pl
There are other cables which also can be used per availability and technical requirements. Do not compromise on connector soldering joints. Poor joints yields in RF loss as well. If you are using antenna switches, make sure you maintain them for best contacts. Remove unwanted coax hops between trx and antenna like keeping a unwanted coax plug joint or a antenna switch which you rarely use etc.
You may see lots of informative articles on arrl.org or elsewhere on grounding. A good ground for antenna tower reduces lightening risk. A good ground in your RF channel like TRX body and Cable jacket reduces return currents and RFI. A good ground for the DC/AC power reduces probability of electric shock.
Over all keep your system clean, even if you are running QRP levels, you will make most out of your available power.
All the best, hope to have a QSO with you soon
DE VU3KPL Kiran