Thick Film Power Resistors at RF Frequencies - Page 1
Forum section: electronics / rf-microwave

author: ocw
author_rank: Regular Contributor
timestamp: February 23, 2020, 02:08:33 am
content: Needing a moderate power load at VHF frequencies I thought that I would try some thick film resistors which are being promoted for RF use. Their specification sheets don’t provide any frequency data so I decided to run my own tests. I chose all 1% resistors and tested the following:1. Caddock MP725-50.0-1% 25 watt TO-252-22. Bourns PWR 163S-25-50R0F 25 watt TO-252-23. Ohmite AP750-50R-F 50 watt TO-263-2While I also purchased some TKH TKH45P50R0FE-TR 45 watt thick film resistors, since they come in a flat SMD package, I didn’t feel that they ae comparable to the others so I’m leaving them out.They were for VHF use, so I used BNC connectors with them. See the attached picture of the Bourns package. They tested better than I expected! I regretted my choice of the BNC connectors as they were probably influencing my SWR readings much as the resistors were. I used my Anritsu MT8222A analyzer to measure their VSWR and prepare their Smith Charts. The 25 watt loads had a VSWR just above 1.2/1 at 500 MHz while the 50 watt Ohmite load was 1.45/1 there. The measurement data details are attached.While the Caddock load had the lowest VSWR, the Bourns load gets my gold star for having just about as good of figures at around half the price of the Caddock ($2.82 vs. $5.91). The 50 watt Ohmite costs $5.19 at Mouser.

author: radiolistener
author_rank: Super Contributor
timestamp: February 23, 2020, 04:44:47 pm
content: VSWR=1.3 at 400 MHz... looks terribleYou can buy RF resistors on aliexpress, RF Florida and other, they are used but works very good - VSWR=1.14 at 500 MHz. Power is about 100W. Their price is just 1 USD

author: ocw
author_rank: Regular Contributor
timestamp: February 23, 2020, 05:41:12 pm
content: QuoteYou forgot about VHF being my primary concern. Let's see your measurement data.I just completed testing the Bourns PWR163S-25-50R0F resistors using SMA and N connectors. They came close to having the same VSWR as when the BNC connectors were used showing that the resistors were the limiting factor, not the connectors. Using the N connectors, the VSWR at 400 MHz was 1.18/1. I purchased those from Arrow for $2.59 each postpaid for overnight delivery. I am not sure that Aliexpress can beat that price for overnight delivery.They are obviously not something for use as a test load for calibration. But, they met my need for handling 5 - 10 watts on VHF frequencies in a small package (TO-252-2). My N and SMA connector data is attached.

author: radiolistener
author_rank: Super Contributor
timestamp: February 23, 2020, 07:21:29 pm
content: Quote from: ocw on February 23, 2020, 05:41:12 pmit has VSWR < 1.03 from DC to 200 MHz.I'm too lazy to measure sweep chart, but if you need I can doHere is link for example:https://www.aliexpress.com/item/32980920678.htmlI bought it from the same seller, but different product link. It was chaper, but now it is not available.

author: ocw
author_rank: Regular Contributor
timestamp: February 23, 2020, 08:30:43 pm
content: QuoteI don't think that a reduction below the Bourns PWR163S-25-50R0F's < 1.09/1 VSWR from DC - 200 MHz is going to matter much for my use. Plus, I suspect that your load is physically larger than mine--I do have size limitations. I didn't need a dummy load--I needed a resistor which will act as a load from DC - VHF.I don't trust manufacturer's data sheets or measurements made on unknown meters. Some inexpensive meter's read 1/1 VSWR until a diode in it starts conducting.The no longer available load which you used was probably better for you and your needs. Likewise, I am happy with my load. You must have gotten a super deal if it was less than the $2.29 each overnight postpaid that I paid for mine (I previously listed the current single unit price).My goal was to present information on a part which was lacking RF data which most people would not consider for use with RF. For many uses, other parts will be better. For some projects, it is a nice option to consider which you can add on to a Mouser or Arrow order. There are only a small number of similar loads available from them.

author: radiolistener
author_rank: Super Contributor
timestamp: February 24, 2020, 05:33:33 am
content: Quote from: ocw on February 23, 2020, 08:30:43 pmI measure it with VNA, so you can believe these results. It was not a cheap SWR meter with diodesBut in order to get so good result it needs to be properly soldered.Quote from: ocw on February 23, 2020, 08:30:43 pmI know... I just wanted to recommend that there are better and cheaper solutionsI recently bought another such resistor, now I got some unbranded "G150N 50W4B" (previously I got "RF Florida 32A1213F C0C") and it also works the same good.G150N 50W4B: VSWR=1.03 at 200 MHz; VSWR=1.13 at 500 MHz32A1213F: VSWR=1.00 at 30 MHz; VSWR=1.04 at 150MHz; VSWR=1.14 at 500 MHzBoth cost me less than one dollar