DG7YBN / Intro to the VE7BQH Antenna Tables -

## Intro to the VE7BQH Antenna Tables

• Understanding the significance of Antenna Temperature in Noisy Environments

With around issue 103 refering to the 144 MHz table, a responsive G/Ta via editable T_earth and T_sky was added.
Around same time more realisic values for average T_earth and T_sky, linked to values in actual releases of the International Telecommunication Union (ITU) were implemented. Why is this a major step forward?

The following excurse holds a number of charts, each showing Antenna G/T as G/Ta in dB/K over increasing environment noise temperature in Kelvin (T_earth) on the horizontal axis. The G/Ta values may look a bit funny as with rising noise the G/Ta hits negative values. However mathematically this is all fine as we divide a value in dB by a plain value, which first has to be converted to dB and than subtracted.

Formula for calculating G/T from dB and Kelvin

When the environments noise temperature goes up, the Antenna Temperture Ta goes up while Gain and pattern shape are static numbers.
As a result, G/Ta, the ratio of static Gain G divided by Ta will get smaller when the environment noise temperature rises.

Let us look at some 432 MHz Yagis to study what happens:

Fig. 1: The chart above shows data for the 6.38 m long M2 432-9WLA.

Now we compare the M2 against a Low Noise type Yagi of roughly same length
and see how increasing environment noise temperature affects the G/Ta of both:

Fig. 2: The chart above shows data for the 6.38 m long M2 432-9WLA
and the G0KSC designed Low Noise concept, 6.21 m long 24 El. LFA.
A Yagi with better G/Ta at lower environment noise temperature will degrade slower when noise rises. A massive gap openes and increases when noise temperature hits the thousands like in urban or city environments.

An experiment of thought:

Now that we have learned that a Yagi with a cleaner pattern degrades with a softer steepness, will a shorter Yagi, which shows a cleaner pattern and thus lower Antenna Temperature catch the 9 wl long M2 Yagi
if only the environments noise temperature goes up far enough?

Fig. 3: The chart above shows data for the 6.38 m long M2 432-9WLA,
and the G0KSC designed Low Noise concept, 3.60 m long 16 El. LFA.
From an environment noise temperature of approximately 2300 K on, the 43 percent shorter Low Noise Yagi takes over in G/Ta.

Can we pull off this stunt with an even shorter Yagi getting the upper hand over the true Longyagi by M2?

Fig. 4: The chart above shows data for the 6.38 m long M2 432-9WLA,
and the G0KSC designed Low Noise concept, 3.60 m long 16 El. LFA,
and the DG7YBN designed Low Noise concept, 2.49 m long GTV 70-13m.

Here we go: From T_earth = 8700 K on the 61 percent shorter GTV also takes over in G/Ta.

Blunt conclusions:
Looking a receiving abilities only
(1) For use in an evironment that the ITU describes as 'residential' one could spare nearly 3 m of boom by choosing a sound Low Noise design Yagi.
(2) in an evironment that the ITU describes as 'city' one could spare grossly more then half the boom.

Now (2) of course would cost grossly 3 dB of punch of the signal transmitted.

So that nobody would suggest cutting down the boom length in ernest. This bit of experiment with numbers and their analysis shall only show the potiential of Yagis with cleaner pattern in noisy environments. So we go back to Chart Fig. 2 and pick a Yagi design from the Antenna Tables with the longest boom we can build and set up with a good G/Ta when we need it to defy environmental noise.