Saturday, November 12, 2016

DIY kits 70W SSB linear HF Power Amplifier For YAESU FT-817 KX3

It's time for a new project.   eBay link to the seller                   

My first impression : High quality PCB ( 100 x 50mm ) , all parts in the kit including 2 power FETs type IRF530N. Capacitors are 10nF instead of 100nF as shown in the schematic but that's not a problem. The 0.5 turns windings are on the PCB by means of the tubes inside the toroids. Construction is easy but a lowpass filter for each band is mandatory. I used the low pass data from W3NQN for a 7 element Chebyshev with standard value silver mica capacitors and T68-6 toroids. The provided wire for the power chocke ( 2 turns ) has a weak coating . It's better to replace it to avoid shortcuts .Start soldering the small parts and end with the transformers. Turn the potentiometer anti clockwise and slowly adjust the bias to aprox. 3.7V. I replaced the FETs by a pair of IOR IRF530N resulting in more stabillety en less input SWR however the input swr is highly effected by the drive power . At 5 Watt the swr mounts to over 1:3... . Best results with this PA are obtained on CW and FM .
The test was in the original configuration at 13.8V and 5Watt FM input from my FT-817. Max. power was 80 Watt . At  3 Watt input the output was about 50 Watt at 20m. but the modulation was as expected distorded in ssb mode at the recommended 2.7V  bias voltage.
At 3.7V ( gate ) bias the modulation is acceptable but far from perfect.  The 150 Ohm resistors over the FETs heat up strongly at full power FM. It's better to reduce the drive power to 2 Watt when working 12V FM/AM . Don't reduce the drive by reducing the Bias voltage for this results in a distorted modulation in SSB mode. Better use a power attenuator : Power att. calculator PA1B
Unfortionally the seller often forgets to add the documentation. I made a copy of the pdf file .
A  You Tube impression by KC2IRV : Overall impression of the amplifier
                                                               Testing the PA 
Visit the blog of John DK9JC for a nice test : Link

Conclusion :  This is a nice kit with acceptable results  for a low price. For best results reduce the drive power to 2.5 Watt.
swr at 5 Watt input : 1:3+.   swr  at 2.5 Watt input 1:1.8 with more than 50 Watt output. This is more than enough for fieldwork.




A second amplifier was build without cooling fan . You can clearly see the 7 element filter for 20m
.The enclosure comes  from eBay :  Link
The heatsink also comes from eBay: Link


 Assembled lowpass filter is also available on eBay :



Sunday, February 14, 2016

Wall Loop 20m.

A stealth solution for apartments .
This max. size open loop antenna for 14 MHz with a totall circumference of 6m. ( 19.68 feet )  is designed to attach to a wall . Not sutable for reenforced concrete walls . It has a no tune bandwidth of more than 300 KHz with a centre swr of 1 : 1.1.  A capacitor was put over the feedpoint to shunt up the feedpoint impedance. The 150 pF parallel capacitor was the best value in my situation ( brick wall ) but can be replaced by a 500 pF tuning capacitor in order to find the lowest SWR at resonance in any setup. First contacts gave good reports in Europe. Although a lot of the hf power is absorbed by the wall, the results are surprisingly good. A test was made with 100 Watts without arcing however 5 Watts are more than enough for indoor use...
The top capacitor is made of twisted pvc coated antenna wire or a piece of 25cm RG-213 coax and is cut for tuning to the frequency . This antenna can also be build on a fibreglass or wooden frame in a diamond shape for field or attic use. Loop circumference for other bands:
30m  : 7m
20m. : 6m
17m. : 4.8m
15m. : 4.2m
12m. : 3.5m
10m. : 3m
Tuning steps :  Change the length or the turns of the top capacitor to tune the loop to the desired frequency then put a 500 pF tuning capacitor over the feedpoint and look for the lowest swr.
You can replace this capacitor by a fixed type.
Follow the thread on QRZ.com :https://forums.qrz.com/index.php?threads/wall-loop-20m.511714/
See de Walloop at DK9JC : https://www.dk9jc.de/blog/equipment/128-20m-indoor-loop-nach-pd7maa 

The capacitor . A piece of RG-213 coax can also be used
Verry good SWR
Feedpoint with 100pF and 47pF in parallel. Notice the line choke.
The loop made by  SP3BGD


SP3BGD feedpoint capacitor for wide range use
The loop at DB9MAT

Disclaimer:I'm not responsable for any damage due to improper reproduction of my projects.

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