Performance Measurements

Introduction

Do you know the status of your radio station? Does it perform the way it is supposed to perform? If you are into weak signal VHF-UHF-microwave work, every dB counts. I will give some hints on how to evaluate your equipment with respect to on the air performance regarding antenna gain, noise figure, G/T etc.

Noise

Noise is everywhere! You have thermal noise, man made noise, galactic noise and so on. Most of the time noise is your enemy, but why not make use of your enemy?

Thermal noise is all around us. Everything with a physical temperature and RF-loss generates thermal noise. That means if you point your antenna at trees with foliage, the antenna will see the thermal noise generated by the lossy foliage. The noise from the foliage will add up to the noise generated by the receiver itself together with other noise sources. Thermal noise is a inherent broadband signal and is measured in dBm/Hz, I.E. power per band width. At an ambient temperature of  290 K (+17 deg. C) the noise power is -174 dBm/Hz. Thermal noise is very predictable and is determined by physical constants and can therefore be used as reference.

Galactic noise is generated by stars, galaxies, our own Milky Way and planets. The galactic noise varies with frequency and is most dominant at lower frequencies and gets hard to detect above the UHF range with amateur equipment. One exemption is the noise from our own sun, it can be detected all the way up to mm-waves even with amateur equipment. Another extraterrestrial noise source is the moon. The noise the moon generates is actually thermal noise. Galactic noise is well documented and can be used as reference. Between the extraterrestrial noise sources we only have the background noise coming from "big bang", birth of the universe. This noise level is quite low and can be used as a reference for low noise (cold sky). The "cold" areas in the sky are larger the the "hot" areas, especially on UHF and up. This means when using a low gain antenna (=wide beam) the contribution from the "hot" areas to the total noise received is low. Pointing a low gain antenna to the sky will receive a noise temperature that is close to the background level.

Man made noise is mostly found in urban areas but sneaks further and further out in rural areas with the introduction of all new data communication, BPL, ADSL, computers etc. Man made noise is not predictable and can not be used for any measurements except for mapping your own noise environment. 

Other noise sources are lightning and static rain. They are of intermittent nature and are, in most cases, of limited nuisance for higher frequencies.

An deeper insight in noise can be found in Ref. [1], [2] and [3].

Prerequisites

In order to be able to evaluate your station with this method, a few things is needed from your station:

A few assumptions have to be made:

 

Measurements and calculations

As mentioned above noise from your surroundings can be measured. By using the natural known noise sources together with the above assumptions some vital parameters of your radio station can be checked.

Noise figure or receiver noise temperature:

Use a small, low gain, low noise antenna with your pre amp mounted close to the antenna in order to minimize losses. Good antennas for this  measurement would be a W2IMU type horn for 23 cm and up. For 70 cm a EIA dual dipole, standard gain antenna would be a good choice.

Point the antenna up into the sky. Avoid pointing the antenna at houses, trees etc. Make the reference reading on your receiver. Avoid pointing the antenna at the sun when getting the cold sky reading, it will give excess noise and give false readings. Point the antenna at the ground and make your second reading. The difference between the both reading in dB is the value you are looking for. This value will be used for the calculations.

Some more to be implemented! 

Antenna gain:

To be implemented soon!!!!!

Tools

VK3UM EME Performance Calculator and SM6FHZ Noise Excel Spread Sheet.

Improvements

EME-systems using an array of yagis can be optimized by using a yagi design with low side and back lobes. The optimization of losses in the system is also a key factor for a well functioning EME-system. Try to find every part that adds loss and lower it as much as possible, every small loss adds up and the sum will be noticeable.

EME-systems with a dish can be optimized by carefully choosing the feed for optimum performance. Here it is a compromise between gain and low noise. Different f/D for the dish will need different feed designs and the f/D also gives some limitations to the noise performance. A deep dish (low f/D) will generally give lower noise then a flat dish. Again, optimize for low loss everywhere including cables, relays, connectors etc.

The noise pick-up of an 50 MHz antenna can be improved by making the radiation pattern as clean as possible. Especially galactic noise can be fought by minimizing the upward and backward radiation of the antenna. If you are pointing directly at the noise source there is not much to be done about it, but try to minimize noise pick-up from other directions. There are several dB's in receive performance to be found.

Links

The EME Performance Calculator by VK3UM can be downloaded at SM2CEW web site.

The following links will give more info about noise and noise sources as well as measurement techniques and antennas.

W1GHZ on line book.

References

[1] IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION. VOL. AP-32, NO. 7, JULY 1984, Natural Radio Noise—A Mini-Review, WARREN L. FLOCK, AND ERNEST K. SMITH.

[2] VHF COMMUNICATIONS -1988, Radio Astronomy for the VHF/UHF Radio Amateur, Hans J. Hartfuss, DL2MDQ.

[3] VHF COMMUNICATIONS -1/1984, Determining the Parameters of a Receive System in Conjunction with cosmic Radio sources, Dragoslav Dobridic, YU1AW.

@ This page is under development and the links may not work as expected. The content of the text will also be updated. Check again soon and they can be updated!

Updated March 25, 2006.             http://home.swipnet.se/2ingandlin/SM6FHZ