++++++++++++++++++++ Date: Mon, 18 Feb 2002 09:20:22 -1000 From: John Buck To: Bill Coleman Cc: Dan Barker , Elecraft Subject: Re: [Elecraft] Power measurement with a 'scope On 2/15/02 7:59 PM, John Buck at kh7t at arrl.net wrote: >>Do not forget to consider that the old Heath cantenna is probably >>about 40 ohms instead of the original 50. Bill Coleman, AA4LR, PP-ASEL, wrote: >I have a cantenna that is 25 years old, and it is still 50 ohms. >Why should it change? This came up on the antenna reflector a couple years ago. Several people noted that their Cantennas were 40 to 45 ohms. Someone theorized that interaction of the oil with the element over long time led to reduced resistance. I got mine out and it measured about 42 ohms. I do not remember the shack temperature at that time. After I got your note, I remeasured mine and got 62 ohms. Temperature in the shack was about 60 Degrees. Removed all coax and switches and got the same reading at the Cantenna terminal. So then I ran 600 watts into it for about a minute and remeasured and the resistance was 56 ohms. Measured a minute later and got 53.8 ohms. Checked the ohmmeter with a 1% 75 ohm resistor and it read 74 ohms. Ran 600 watts for 2 minutes more then measured 48 ohms. 5 minutes later it is back up to 53 ohms. Now I am confused. I do not know why I got the original 42 ohm reading. Other than temperature sensitivity I have no clue as to why the earlier reading was so low. I believe that my readings are consistent and the connections are good. It is unlikely that my original reading was made hot. One thought is that my Cantenna had not been used much at all in the years prior to my original 42 ohm reading. It had been very rarely used at power levels of 100 watts or less because I had a dummy load in the tuner. Since that time I started using it with an SB220 to tune up after band changes. So it would get about 10 seconds of 600 watts a few times a week. I think the moral of this story is that the Cantenna is not a precision 50 ohm load. Aloha, John KH7T ++++++++++++++++++ Date: Wed, 27 Nov 2002 20:18:15 -0700 From: Larry Benko To: Jeff Burns Cc: elecraft_list Subject: Re: [Elecraft] Cantenna vs. Oak Hills Research RFL-100 Jeff, If you want a quality dummy load for 100W I would look at the surplus ones used in the cell phone industry. I see them all the time at hamfests for $15 to $25. They look like a heatsink that is about 3"x3"x8" and always have an N connector on the end. I bought several a year or so ago and made some measurements on them. I don't know what the ratings are but they will take 100W forever and get barely warm. Better yet they actually look like 50.0 ohms. Measurements made on a $40K Agilent network anaylzer shows SWR less than 1.02 thru 200MHz and 1.1 at 1 GHz. They have absolutely no liquid inside them so spillage is no problem. Any dummy load kit which actually may measure 50.0 ohms is very likely to be slightly reactive. Not really a big deal but these other dummy loads are definitely superior. You will need to buy a N to UHF adapter. 73, Larry, W0QE Jeff Burns wrote: > I have a Heath Cantenna HM-31-A RF load resistor I purchased when I built my > K2. To do the K2 alignment I ended up using a network-terminating resistor > instead because the Cantenna measured 52 ohms at DC. > > Now I am thinking of getting a KPA100. To use the Cantenna at 100W I suppose > I need to put some oil in it. The thought of keeping an oil filled can for > just a 100w rig does not excite me much. Especially with a two year old in > the house. I am thinking of selling the Cantenna and getting a the Oak Hills > Research RFL-100. > > Any comments on the Oak Hills RF load? Am I being short sited moving down in > power handling capability? > > Jeff Burns > AD9T ++++++++++++++++ Date: Wed, 27 Nov 2002 22:18:15 -0600 To: "Jeff Burns" , "elecraft_list" From: Tom Hammond =?iso-8859-1?Q?N=D8SS?= Subject: Re: [Elecraft] Cantenna vs. Oak Hills Research RFL-100 Jeff: >I have a Heath Cantenna HM-31-A RF load resistor I purchased when I built my >K2. To do the K2 alignment I ended up using a network-terminating resistor >instead because the Cantenna measured 52 ohms at DC. That's a small error... the 52 Ohms should have worked just as well... actually it should have better matched the character impedance (52 Ohms) of the Coax anyway. >Now I am thinking of getting a KPA100. To use the Cantenna at 100W I suppose >I need to put some oil in it. Nah... the Caborundum resistor in the Cantennas easily handles 100W with no additional cooling. There are derating curves on the side of the can... check 'em out. >The thought of keeping an oil filled can for just a 100w rig does not >excite me much. Especially with a two year old in the house. Been there... 73, Tom N0SS ++++++++++++++++