Parallel wiring of non-inductive (globar) resistors?
I'm making a new cantenna for the shack. I've been looking through the surplus websites for globar resistors. The rig puts out about ~130W or so; I'm thinking that a 200W globar resistor submerged in transformer oil would handle tune-ups without much strain.
It's hard to get a 50 ohm globar resistor with just the right wattage. In order to get a sufficient amount of wattage capacity, should I buy two 100 ohm resistors, or four 200 ohm resistors etc. and wire the resistors in parallel, or look to get a single 50 ohm resistor? I have to keep in mind the dimensions of the paint can. I suppose I could go for something larger like a big metal pot if I need more room for multiple resistors.
Especially for HF, paralleling several resistors should work fine.
The methods you wish to use will be good. Make the ground end of the resistor pack as large of a conductive path as you can. That will help minimize inductive reactance on the high frequencies. If you wanted to you could even use twenty 1K ohm resistors. It's really up to you and what you can obtain.
A very long time ago, I tried LOTS of 2W carbon resistors in parallel. It was horrible anywhere near 30 MHz but was OK on 80m and 40m. There's this little thing about reactance that you want to avoid. Keep the number of resistors to a minimum, and the interconnection lead lengths as short as possible. OTOH, I have a 90W globar collecting dust. I bought two to refurb a Cantenna, and that one was an ohm or so off of 50Ω. I'm sure it'd like to be used...
Originally Posted by AB2T
Submerging the globar in oil will increase it's wattage dissipation about x5, so a 50W resistor should handle around 250W submerged in transformer oil. Other oils have a bit less of a multiplier so try to get transformer oil. Sometimes your local electrical company will give you a gallon for free. It won't hurt to ask.
The old Cantenna resistors were 200W and had a derating curve up to 1000 watts printed right on the can.
73 de N4CR, Phil
Most electric company service centers acquire transformer oil in 55-gallon drums and usually have several drums at any one time. Frankly, it costs the company more for all of the paperwork, etc., to sell a gallon than the oil actually costs. Therefore, if you ask nicely, most service centers will just give you a gallon rather than have all that "bother" to sell you a gallon. Besides, they use so much and really don't keep track of how much oil is used for each job.
There is a differnt way to make a "flat" load all the way to 450 mhz.
For less than $10 you can get a surface mount 50 ohm non inductive 150 watt rated load resistor to mount to a heat sink of about 6"x6' or larger that will more than take your 130 watts for many minutes at a time.
With a small fan even much longer.
How much time would you need at 130 watts? Not much.
Using good UHF construction ways and tricks, you can extend the load's flat match all the way to 450 mhz.
Build with an N type connnector so it stays flat at 450 and use a UHF to N adapter for all other equippage use.
This is my shack load and now a standard for all testing of cables, connectors, coax relays and transmitter/amplfier power and feed lines all the way out to the antennas.
Weight about 1 lb and about half the size of a 2 meter solid state amplifier and use in any position inside or out.
The Heath HN-31 Cantenna uses Kanthal/Globar 886SP. They're rated 90W continuous in free air. MFJ has it as #115-1500 (used in their MFJ-250). I bought two for $20 (ebay) when I rebuilt my HN-31. I have one left that's not doing anything (hint to OP).
Originally Posted by N4CR
That's how I obtained a gallon of xfmr oil. It took a few phone calls to get connected to the supervisor for the transformer shop. Since their pumps have a very high flow rate, it was tricky to get ONLY one gallon!
Originally Posted by K9STH
My dummy load uses four 50Ω / 400W resistors (Anaren #RFP-400-50R) in series-parallel, mounted to two heatsinks (140 2in each), with the fins oriented vertical, and the mounting surfaces face-to-face. A 100 cfm fan blows air thru the fins (via careful ducting). Wide copper busses provide relatively low XL interconnects. VSWR is below 1.5:1 up to 50 MHz, as indicated on a VNA. All parts were scrapped from my former employer -- cost = zero.
Originally Posted by KM3F
Thick film power resistors also work pretty well for dummy loads. They mount easily to heat sinks.