home made PA and load - Page 1
Forum section: electronics / projects

author: joeqsmith
author_rank: Super Contributor
timestamp: March 23, 2015, 04:36:27 am
content: While I was working on increasing the bandwidth of my old current probe, I needed an amplifier along with a few other parts.I want to build a dummy load that can handle a few Watts and good to a few hundred MHz. I cut a heat sink up for it and plan to sandwich the parts inside.The load is made from a RM250 50ohm fixed film stripline resistor.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 23, 2015, 04:42:07 am
content: This is what it looks like together. The N is the input and the SMA is a 40dB tap. Looking at it with the network analyzer it is pretty flat out to 150MHz. Last picture is zoomed in to 0.1dB/div.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 23, 2015, 04:51:43 am
content: Looking at the load with the VNA, I measured10MHz - 49.6 ohms50MHz - 49.6 ohms200MHz - 49.6 ohms500MHz - 49.21 ohms650MHz - 48.82 ohms

author: joeqsmith
author_rank: Super Contributor
timestamp: March 23, 2015, 04:55:12 am
content: The heatsink for the PA. Uses two N launch connectors. The lowered areas are for the transformers.

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 23, 2015, 01:45:00 pm
content: It looks good.Why didn't you use a bigger heatsink seeing the resistor can take it.When do we get a look at the PA?

author: AF6LJ
author_rank: Unknown
timestamp: March 23, 2015, 02:22:50 pm
content: Looks good enough.I suspect you have no intention of running more than about 25 or so watts into that load given the size of your heat sink.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 23, 2015, 11:25:20 pm
content: Amplifier is based on a Motorola app note. Looking for 100MHz 50W average or so. Amp's sink must be forced air cooled.I used 6 1206s for the divider. You can see them in the first picture. They would dissipate around 1/2W with a shorted output. Should handle 3/4W. This is the limit.Spent a little more time trying to flatten it out. Around 0.3dB now. Last picture is 10dB/div. 0dB center, 10dB offset. Red is basically at -40dB in far left. Cursor is at 300MHz where it gets pretty bad.Using a small fan with 50VDC applied and half the sink, temp gets to about 80C. With the sink together, around 50C.

author: dom0
author_rank: Super Contributor
timestamp: March 23, 2015, 11:57:22 pm
content: Might be a stupid question: at .05 dB/div and 100 MHz — don't you get quite some flatness issues by cables, connectors etc. alone?

author: joeqsmith
author_rank: Super Contributor
timestamp: March 24, 2015, 01:25:30 am
content: I have to calibrate the system. Below a GHz, things are not so bad.I connected the load to the amplifier that I modified to test the current probe. At 50MHz, I can get 700mV RMS after the attenuator. Or, about 100W. Even with the small fan, sink reached 107C. This amp after changing the transformers can now put out about 12W at 80MHz and 2W avg at 120MHz.Here is the application note for the amplifier I am putting together:ftp://ham.hr/Hardware/transverter/Wideband%20AMP/Motorola-%20Wideband%20RF%20powe%20amplifier%20AR-313.pdfI am cheating and bought a kit. There's no heatsink or connectors but the rest is there.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 24, 2015, 01:49:14 am
content: Putting the kit together...

author: AF6LJ
author_rank: Unknown
timestamp: March 25, 2015, 12:47:32 am
content: The copper heat spreader will make a huge difference in thermal stability.Forced air cooling should work okay though. I'm all for basic kits, there is no need to reinvent the wheel.Looks good, keep us posted on how it is working.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 26, 2015, 01:18:25 am
content: Quote from: AF6LJ on March 25, 2015, 12:47:32 amThe transformers are tight against the heatsink with a small amount of thermal compound. So far, seems to be fairly stable. Sink would need to be a lot larger to push it.Made a cover for the heatsink out of some circuit board material.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 26, 2015, 01:21:58 am
content: Shown running 110MHz with 300W supply. The other amp runs out of steam to drive it. Still 72 Watts out of it is good enough.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 26, 2015, 01:35:02 am
content: Turning down the frequency a bit to get some more drive. Shown at 100MHz'ish.68.3 Vrms or 93.3 W, or 49.7dBm. Ran it like this for about an hour. The amplifier's heat sink was fine but the tiny heatsink I used for the load even with 110 CFM fan is getting a bit on the hot side.Also shown connected to the spectrum analyzer with a 10KHz span and 150Hz RBW. 0.5dBm of error between the scope a network analyzer. Did not allow it to warm up.Not a bad little kit.

author: AF6LJ
author_rank: Unknown
timestamp: March 26, 2015, 04:03:53 am
content: That heatsink is a tad small for a hundred watts.My commercial dummy load (DC-1GHZ at 100W) is rated for 100W continuous is about 3"X3"x 111" long it will get up to about 40-50 C after an hour or so.Your home made load is doing quote well.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 29, 2015, 12:41:15 am
content: Quote from: AF6LJ on March 26, 2015, 04:03:53 amI looked at buying an attenuator and load to test the amplifier. They should be more accurate than what I built.

author: AF6LJ
author_rank: Unknown
timestamp: March 29, 2015, 04:56:52 pm
content: Quote from: joeqsmith on March 29, 2015, 12:41:15 amYou have to ask yourself; do you really need that level of precision?The difference between a 50, and 52 ohm load and one that might drift a little in value when its hot is very small. I don't know what you are using this for but I doubt the operating conditions will be that well controlled.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 29, 2015, 05:20:23 pm
content: I do not.The only time I have needed any sort of power in this sort of frequency range was to test the mods I made to my P6042 current probe. Did I need a 100MHz current probe? Not at all. It was fun to see if could be done.When I build something I plan to keep, I will normally take the time to make it the best I can with what I have.

author: AF6LJ
author_rank: Unknown
timestamp: April 01, 2015, 02:42:52 pm
content: Fun project nonetheless.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 04, 2018, 10:38:42 pm
content: Why not use a standard load and a resistive tap that could also be used for other measurement purposes ?Or, if power is too much, a standard attenuator, the a tap, then a load. Its more versatile.Like this:https://electronicprojectsforfun.wordpress.com/rf-module-gallery/the-rf-splitter-and-combiner-gallery/a-resistive-tap-attenuator-for-rf-measurements/