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Review of PA150 HF Linear Amplifier

T K Mani / VU2ITI and Niras C V / VU3CNS

The Linear Power amplifier PA150 designed and marketed by OM Krish VU3NKK is of great attraction by many HAMs in India. This Linear is the one professionally assembled and is ready for sale to HAMs for 7200 Rs. The price is very attractive, and there are many reports form hams that this linear is performing well. There is a blog page for this device: homebrewcorner.blogspot.in/p/pa-150-hf-linear-200w-amplifier.html

PA150 HF Linear Amplifier
PA150 HF Linear Amplifier

I was attracted to this linear because of (a) this is the first commercially available Indian made linear amp for ham radio and (b) the manufacturers test report about the rig. The power efficiency is claimed to be unbelievable for this class AB amplifier. Theoretical efficiency of any class AB amplifier is less than 78.5 % where as this class AB amplifier efficiency is claimed to be more than 80% and in some cases more than 100 %!. I had contacted the ham who made this linear and he explained me that what he had published about the linear is what he had observed while testing. Specification of PA 150 shown below is based on the details given on the PA150 Web Page:

Specification of PA150
Table 1. Specification of PA150 as obtained from the web

Not convinced with his explanation, I thought of testing the linear. Luckily, VU3WIJ, OM Shaji agreed to offer his PA150 for testing. VU3CNS, OM Niras managed to get this from him.

We together planned to test the linear at the incubation centre in the Incubation Centre of KMEA Engineering College. The test equipments used are as shown below:

Test equipments used
Table 2. Test equipments used

The test setup was as shown below:

Test setup for testing the PA150 HF Linear Amplifier
Fig. 1. Test setup for testing the PA150 HF Linear Amplifier

Test procedure and test Results

The transceiver FT817 was used to drive the PA. SSB PeP output of FT 817 is 10 watts and 5 watts in CW mode on all Bands. First we tested the output wave forms and the frequency spectrum of the output of FT817 by loading it into dummy load MFJ250. Output waveform is clean and free from harmonics. Please see (Fig. 1. A to D). We used 1:10 attenuation probes for all measurements.

Input and output waveforms
Fig. 2. Input and output waveforms (See text for details)

Next, FT817 output is connected to the input of PA150 and dummy load is connected to the output of PA150. It wais then observed that the input waveforms became distorted (Figure 2 E to H). The output was also found distorted (Fig. 2. I to L). It was observed that the distortion increases when the drive increases. It was also observed that the output wave form is very clean for 21 MHZ band and 28 MHZ band. However, RF the power output was found reduced for 21 and 28 MHz, may be due to insufficient drive from the FT817.

Output and Input Spectrum

We used 10000 MHz Aplab Model SA3011 Spectrum Analyzer for observing the frequency spectrum. Figure 3 shows the input and output frequency spectrum of PA150. The figures clearly indicate that the output is rich in harmonics. It was also noticed that there is considerable reduction in harmonics in the 21 MHz and 28 MHz bands.

Frequency Spectrum of the Input Signal
Fig. 3a. Frequency Spectrum of the Input Signal.

Output Spectrum of PA 150 in 7 MHz High Power mode
Fig. 3b. Output Spectrum of PA 150 in 7 MHz High Power mode.

Output Spectrum of PA 150 in 7 MHz Low Power mode
Fig. 3c. Output Spectrum of PA 150 in 7 MHz Low Power mode.

Input Spectrum at 14 MHz
Fig. 3d. Input Spectrum at 14 MHz.

Output frequency Spectrum at 14 MHz Low power mode
Fig. 3e. Output frequency Spectrum at 14 MHz Low power mode.

Output frequency Spectrum at 28 MHz
Fig. 3f. Output frequency Spectrum at 28 MHz.

Harmonics Distortion

Harmonics distortion was observed on all bands. This may be due to the fact that there is no harmonic filter at the output stage of this amplifier. To get an idea of the order of harmonics present in the output, Table 3 was prepared for the 7 MHz band. It shows that noticeable harmonic frequencies are present.

Harmonic contents in the output spectrum
Table 3. Harmonic contents in the output spectrum.

Power Efficiency

Power efficiency of PA150 is calculated by using the formula:

Power efficiency = Output Power ⁄ Input DC Power × 100%

Table 4 shows the consolidated results. Output power is measured by using the ‘Ameritron AWM-35’ power meter and also by using the Digital Storage Oscilloscope. It is noticed that the ‘AWM-35’ indicated lower reading than the power calculated using the DSO. The DSO being a calibrated one we can trust the power calculated using DSO rather than the ‘Ameritron AWM-35’.

Maximum power efficiency obtained was 60%. It was observed that input power (from FT817) reduces at higher bands. Hence we could not load the PA150 to its full power on high frequency bands (21 and 28 MHz).

Power efficiency measurement of PA150
**Measurement error noticed
Table 4. Power efficiency measurement of PA150.

Modulation

All the tests as explained above were conducted on CW. The performance of the rig is carried out by using SSB signal and the output waveforms are as noted below

Haalow & Whistling
Fig. 4a. “Haalow”.                     Fig. 4b. “Whistling”.

Conclusion

We had attempted test one PA150 Linear Amplifier. Our DSO, Spectrum analyzer and DMM are calibrated and accuracy is sufficient for this test. However, the Ammeter used to measure the DC current is not very accurate and can cause small measurement errors. Hence the power efficiencies as mentioned in table 4 may change slightly (±5% approximately).

Since there is around 18% harmonics distortion, the maximum effective power output in the desired frequency will be less. It is also suggested to use harmonics filters at the output stage in order to prevent harmonics radiation.

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