Having several of these on-hand I decided to take a quick look at its apparent performance - with the general specifications for this device being listed below for your convenience:
- Frequency range: 1 MHz-300 MHz
- Gain: 23dB (or 24.5 +/- 1 dB, depending on source)
- Gain flatness: 1 dB
- Noise figure: 3.8dB (frequency not specified)
- Input/Output VSWR: <=1.5:1
- Power output (1dB compression): +22dBm
- 3rd order Intercept: +37dBm
- Current consumption: 155mA (voltage not specified)
- The data sheet from "API Technologies" shows the input/output return loss as 1 dB - clearly a typo.
- The maximum voltage rating is all over the map: Some versions of the data sheet show a maximum of 20 volts, others show 24 volts. The units that I have clearly show the voltage rating as being "+15/24Vdc" and the equipment from which it was pulled provided 24.0 volts.
A quick test:
Curious about a few aspects of these amplifiers I decided to test it with my DG8SAQ Vector network Analyzer, checking its gain versus frequency in the input matching (e.g. S11) - the results being displayed in Figure 2, below:
- The gain at 285 kHz was still above 21dB - a bit more than a 2 dB drop from the peak gain. The input VSWR was still below 1.5:1.
- The gain at about 200 kHz was around 18.7dB
- The gain at about 173 kHz was around 16.8 dB and the VSWR had increased to about 2.7:1.
- The gain was above 22dB at 400 MHz
- The gain was about 21 dB at 440 MHz
- The gain was around 18dB at 500 MHz.
Gain versus operating voltage:
Figure 2 was captured with the unit operating at 18 volts and readings were taken at lower voltages, comparing the gain - but your mileage may vary:
- Gain dropped by approximately 0.1 dB at 15.0 volts.
- The gain was about 0.2 dB lower at 12.0 volts than at 18 volts.
- The gain was about 1 dB lower at 8.0 volts than at 18 volts.
- The gain was about 5 dB lower at 5.0 volts than at 18 volts.
- The amplifier began to exhibit signs of low-frequency instability below 5 volts.
Although not directly measured, one should expect the maximum output power (P1dB) and the intercept point to drop below the specifications when operating it from lower than 15 volts: The amplifier is likely to be perfectly usable in the 12-14 volt range, but it's likely marginal at 8 volts and below.
A peek under the hood:
Popping the top cover, we see this:
A look inside the QB-300 amplifier: The input and output is on the left and right sides, respectively.
Click on the image for a larger version.
It is immediately apparent that this is not a run-of-the-mill consumer device: Rather than a circuit board, the unit is built onto an alumina substrate with both soldering of components and spot welding of wires being used. Two RF transistors are obvious: The black, 3-lead device near the upper-left corner and the white ceramic device marked with "Q-21" just to its right. The rest of the components are likely related to feedback/equalization as well as regulation of the operating and bias voltages for the RF devices.
Clearly, it's not hermetically sealed or conformally coated, so weather protection is certainly warranted if this were to be operated outside.
Uses for this amplifier:
This amplifier was designed as a general-purpose gain block in the HF-VHF range, but it is likely useful into the low UHF range meaning that it should work from the 630 meter amateur band (on the low end) into the 222 MHz - and possibly the 70cm - amateur bands on the high end.
For general HF (amateur radio) amplification purposes, it should be an excellent performer - provided that one keeps in mind that it's gain may be a bit too high in certain applications in that a signal input level of a around -5dBm will push it into overload - and off-air signals of this strength might appear from:
- Local AM broadcast stations. Especially on a long wire antenna (longwire, rhombic, end-fed half-wave) these signals can, by themselves, overload the amplifier if you live anywhere near a transmitter. A simple high-pass filter can effectively reduce such signals and prevent overload.
- High-power shortwave stations. On a good antenna, signals on the 49, 41 and 31 meter band can be extremely strong in Europe and some parts of the U.S.
In short: If you hear more noise when you connect your antenna system to your receiver system than when you connect it to a dummy load, you are OK - but if you can't hear the difference, your system will not be sensitive enough to hear the weakest signals.
For receive-only purposes it is often the case that with a low-noise amplifier, a good, quiet (in terms of noise) receive antenna will not need to have much gain from the antenna itself - and if the gain is low, you are less likely to intercept enough absolute signal power to overload the amplifier. Here are just two of the many possible examples of antennas to consider:
- Small receive loop. This type of antenna - usually around 3 feet (1 meter) diameter for MF and HF use can offer local noise rejection as well as the ability to null signals from directions broadside the plane of the loop. This type of antenna will have negative gain (e.g. less than 0 dBi) but its performance can be quite good with a decent, low-noise amplifier like the QB-300. For an antenna like this, one would place the amplifier at the antenna to minimize cable losses.
- Beverage on the ground. Also known as the "BOG" antenna, this is simply a wire - as long as possible - laying on the dirt and working against a good (and electrically quiet) ground consisting of one or more ground rods and counterpoise wires and its feedline electrically decoupled (with a "current" balun) to prevent noise from the shack from being brought to the antenna. This antenna - mostly useful in rural areas - is reported to work well overall despite the likely "negative" gain. As with the receive loop, it's best to place the amplifier at the antenna feedpoint.
It should go without saying that any amplifier (or receiver) connected to a large antenna should be preceded by adequate lightning protection to prevent damage to the amplifier from wind static/discharge and nearby lightning strikes as depicted in Figure 4, below. Such filtering should be placed after any filtering that might precede the amplifier.
Decent protection can be had with four ordinary silicon diodes - two series pair connected anti-parallel (back-to-back) with a bleed resistor (4.7-100k) to shunt voltages above about 1.2 volts. It's worth noting that the amplifier itself would already have overloaded before signals can a high enough level to cause the diode protection to conduct and cause distortion!
Provided that one avoid excessive signal input level, it can also be used as the basis for a receive multi-coupler. For example, following the amplifier with an 8-way RF splitter - which, itself, will have a loss of around 10dB - the overall gain will be in the range of 14dB while preserving the system's overall noise figure to allow reception of weak signals on the higher HF bands.
This page stolen from ka7oei.blogspot.com