Owen – Page 97 – owenduffy.net

Designing high performance VHF/UHF receive systems – Part 4

Finding transceiver Teq

We have explained how to calculate Teq from Noise Figure, but most transceiver specifications do not give Teq or Noise Figure directly, in fact they don’t really contain sufficient information to reliably calculate Teq or Noise Figure.

Credible equipment reviews might provide an estimate of Noise Figure or Teq.

The best approach is to directly measure Noise Figure using a known noise generator and the Y Factor Method.

Continue reading Designing high performance VHF/UHF receive systems – Part 4

Last update: 19th June, 2014, 10:00 PM

Designing high performance VHF/UHF receive systems – Part 3

Relationship between Teq and Noise Figure

In the last part, the meaning of the equivalent noise temperature of an amplifier was given.

Whilst you will find that working in Teq has advantages for this analysis, amplifier specifications may not give Teq, but may give Noise Figure.

Continue reading Designing high performance VHF/UHF receive systems – Part 3

Last update: 19th June, 2014, 10:01 PM

An inexpensive current limiter for flashing and initial testing of ESCs – Mk II

There is a risk of damage when flashing ESCs. It accrues from the fact that ESCs have a three-legged H bridge and if a high and low FET are turned on simultaneously, damaging currents may flow. In fact, this can be an issue if the FETs are on together for just microseconds on each PWM cycle. Loading the wrong hex module is a recipe for disaster, it may turn on FETs in an unexpected way.

So, for safety, the ESC should be powered from a current limited power supply during flashing and initial motor testing.

In a process of continuing development, this article describes a variation on the inexpensive current limiter for flashing and initial testing of ESCs – Mk I.

Continue reading An inexpensive current limiter for flashing and initial testing of ESCs – Mk II

Last update: 21st February, 2014, 7:33 PM

Designing high performance VHF/UHF receive systems – Part 2

G/T is defined as the ratio of antenna gain to total equivalent noise temperature.

For clarity, lets define those terms.

Gain

Gain of an antenna is defined (IEEE 1983) as the ratio of the radiation intensity, in a given direction, to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically. (Isotropically simply means equally in all directions.)

Continue reading Designing high performance VHF/UHF receive systems – Part 2

Last update: 19th June, 2014, 10:01 PM

Cooling an IC2200H

I have an IC2200H mounted on my operating table with 25mm clearance above the radio and ample room for convection currents to assist in heat removal. It is concerning that the case temperature reaches temperatures that are not safe to touch, temperatures in excess of 75° (55° above ambient) have been measured and that has not triggered the internal temperature protection… so it could get hotter still!

Whilst it might take a while for the radio to reach high temperatures, in the long term, it must dissipate around 139W when transmitting on HIGH power setting and at ambient temperatures as high as 35° in the shack. (Rated input is 15A at 13.6V for 65W out, leaving 139W of heat to be dissipated.)

This is one of those high power mobile radios that advertises no fan as an advantage, but it is clearly not up to the task!

The objective of this change is to keep the external parts below 60°, the (ASTM standard C1055 1999) 5 second human skin burn threshold.

Continue reading Cooling an IC2200H

Last update: 15th May, 2015, 5:45 PM

Mag loop or radiating dummy load?

There is a seemingly endless series of articles on small transmitting loops on the cheap.

(eHam 2014) is another, it describes a so-called magnetic loop for transmitting on 14.2Mhz using 4.57m of 2.6mm copper wire for the main loop. The author reports the bandwidth of the finished antenna as 100kHz. One of the claimed benefits is that with such wide bandwidth, a variable tuning capacitor is not required.

Continue reading Mag loop or radiating dummy load?

Last update: 5th February, 2016, 5:22 PM

Designing high performance VHF/UHF receive systems – Part 1

G/T

A metric that may be used to express the performance of an entire receive system is the ratio of antenna gain to total equivalent noise temperature, usually expressed in deciBels as dB/K. G/T is widely used in design and specification of satellite communications systems.

G/T=AntennaGain/TotalNoiseTemperature 1/K

Example: if AntennaGain=50 and TotalNoiseTemperature=120K, then G/T=50/120=0.416 1/K or -3.8 dB/K.

Continue reading Designing high performance VHF/UHF receive systems – Part 1

Last update: 25th February, 2014, 10:13 AM

VK2OMD ambient noise measurement 144MHz – 20140217

I made a measurement of ambient noise on 144MHz this morning using the technique described at (Duffy 2009).

First step is to recheck the NF of the receiver. The TS2000 is getting a little tired, NF=8.3dB.

The technique calculates ambient noise from the variation in receiver output noise of a receiver of known Noise Figure with the insertion of a known input attenuator. The receiver output noise was measured using NFM (Duffy 2007) which allowed integration over 20s for high resolution measurement.

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Last update: 5th March, 2014, 12:02 PM

VK2OMD G5RV with tuned feeder – line loss

The G5RV Inverted V antenna system at VK2OMD is fed with 9m of home made open wire transmission line using 2mm diameter copper wires spaced 50mm giving a line with characteristic impedance of 450Ω. (Varney 1958) described the tuned feeder configuration of his popular G5RV antenna system.

Continue reading VK2OMD G5RV with tuned feeder – line loss

Last update: 27th February, 2014, 4:36 AM

BLHeli v11.0 on APM X450 quad

I had some Skywalker 40A with BLHeli v11 spare from the tricopter project, so I have put them on the APM X450 quad for a trial.

The BLHeli beacon is really annoying, so I turned it off. Otherwise it is a pretty basic Multi BLHeli configuration.

The ESCs were throttle calibrated using APM’s all at once procedure… but it didn’t work as expected, the motors remained stopped when armed. A lot of fiddling around to find some parameter changes did not work until power was cycled. Set MOT_SPIN_ARMED=200, THR_MIN=220.

Initial tests in stable, alt hold and loiter modes have been good.

Last update: 16th February, 2014, 5:51 PM