Heathkit "Cantenna"

[Andy1845c]

I received this as a Christmas gift from my dad. I have been wanting to get into ham radio for a while, and am working on getting a rig set up.The last thing I need is another iron in the fire, but oh well

I am confused on what the 50hm: resistor should read with an ohm meter. I know, that sounds stupid, but it reads 77.1hm: on mine. I checked some new resistors with the meter, and it seems okay, so I suspect the 50hm: resistor could be bad. I googled this problem and came up with mixed information. Someone said the resistor could have been overheated and be ruined, some say these can't be hurt. Some say that running them in the wrong type of oil can make them read high. Can anyone tell me what is correct?

I am struggling to understand impedance. The can reads 50hm: impedance. That dosn't simply come from the 50hm: resistor, does it?

Are there any ham radio gurus out there that can lend some advice on this. Will it be any good to tune my radio the way it is?

 

[Nigel Goodwin]

My first thought is 'why do you want a dummy load'?.

I would expect it to read 50 ohms on a meter!

 

[Hero999]

Yes it should read 50hm:

At first I thought it could be a simple attenuator but then after looking at the schematic it's an attenuator and rectifier used to monitor feild strength.

 

[stevez]

The load should read 50 ohms or very close to it - within 5% would be my thought. I have one and it reads 49 ohms.

A dummy load is quite useful in transmitter work. It allows one to operate the transmitter without radiating much and also it allows the transmitter to work into a known load. I have one and before puttting any transmitter on the air I first run it into the dummy load to see if it behaves as I'd expect.

A bit of info - the Cantenna and other similar "dummy loads" are essentially a 50 ohm resistor that is non-reactive or non-inductive - and that can handle the power delivered to it. The resistor in the Cantenna is immersed in one gallon of oil. This allows it to handle 1 kw of power for quite some time.

There are other dummy loads available that won't handle quite so much power - and those are actually quite useful as many modern transmitters run at 100 watts or less. Yes, you can purchase non-inductive resistors and make your own. Be careful in that the advertized dissapation is at 25 C - and drops a great deal as it warms up. Still, those can be low cost ways to build your own. Keep in mind that the resistor and the assembly must be designed to be non-inductive at the freuqency of use. The cantenna works up to 150 mHz if my memory is correct. There are a number of home-brew designs that work well into UHF regions.

 

[stevez]

Andy - I neglected to mention this - I'd encourage you to join a local ham radio club. Take a look on the ARRL website for one near you. While this forum is a great resource - having someone close at hand to show you things is worthwhile as well.

 

[JimB]

I am confused on what the 50hm: resistor should read with an ohm meter.

As commented elsewhere, it should be 50 ohm.

I know, that sounds stupid, but it reads 77.1hm: on mine....so I suspect the 50hm: resistor could be bad.

Yes the resistor could be bad,
or..
There is a bad connection somewhere in the can. Maybe the clamps on the ends of the resistor.
or..
Someone has changed the resistor to a 75 ohm type.
In days gone by, 75 ohm was commonly used as the load for a transmitter. Don't forget that a dipole has an impedance of 75 ohm.

I am struggling to understand impedance. The can reads 50hm: impedance. That dosn't simply come from the 50hm: resistor, does it?

Quick answer - yes 50 ohm is just the resistor.
Longer answer - at higher frequencies, the size and shape of the metalwork around the resistor has a big effect on the impedance and hence VSWR.

JimB

 

[Andy1845c]

Thanks everyone for your replys.

stevez - I'll look into joining a ham radio club. I'm not sure there is one around, but its worth a look

 

[Russlk]

I have a Radio Shack Cantenna, it measures 51 ohms. It would be worthwhile to look inside to see if there is a bad connection, or the resistor may be cracked.

 

[Andy1845c]

I have a Radio Shack Cantenna, it measures 51 ohms. It would be worthwhile to look inside to see if there is a bad connection, or the resistor may be cracked.

I'll take a look right now...

 

[Andy1845c]

Well, I pulled the resistor out. It has 50ohms printed right on the side. (What is that thing made out of anyway? Is it ceramic?) Anyway, its not cracked that I can see, and still reads 77 ohms all by its lonsome self. So I take it its bad.
Can anyone take a guess at how it went bad? Age? Heat?
How about a replacement? Any ideas?

 

[Sceadwian]

Have you tried to test it under any substantial load? The can says this thing is rated at 1kw, testing the resistance on a meter is completly pointless as the load is so low. Who knows, maybe the 70ohms means the resistor leads are actually dead shorted in the oil bath and the resistance is coming from the burned off resistor material? Testing it under load will show you pretty well what's gonig on with it.

 

[Andy1845c]

Have you tried to test it under any substantial load? The can says this thing is rated at 1kw, testing the resistance on a meter is completly pointless as the load is so low. Who knows, maybe the 70ohms means the resistor leads are actually dead shorted in the oil bath and the resistance is coming from the burned off resistor material? Testing it under load will show you pretty well what's gonig on with it.

No, I have not. How would I go about that? Do I need a special meter, or could I just run some current though it and check it with the DMM I have?

There is no oil in it at the moment, it has had oil in it though. I checked the resistor alone, so the oil bath and connections are pretty much ruled out.

 

[Nigel Goodwin]

I'll try asking again? - what are you wanting to use it for?, why bother testing and repairing something unless you've got a need for it?.

 

[stevez]

Are you sure your meter is accurate? It ought to read about 50 ohms - power level won't change it.

 

[Andy1845c]

Are you sure your meter is accurate? It ought to read about 50 ohms - power level won't change it.

Yeah, pretty sure anyway. I've tried two differnt ones, and have checked brand new 1/4 watt resistors with mine and they come out within 5%.

Nigel - I've never used a dummy load before, but every ham I have talked to says I will want one. As I understand, it is hooked up in place of the antenna and allows for tuning of the transmitter without going on the air. Learning how to use it will be my next step, but I figure I need to have it working right if I expect to get any kind of accuracy when I do use it. I got my radio (Tempo One Transceiver) a long time ago, and I have never tried the transmitting side of it. I've sure it is way out of adjustment.

 

[JimB]

OK have a look at the picture below.
Three resistors removed from RF Power meters many years ago.

The long one is just under 10" in length, it should be 50ohm, and I measure it at 49.3 ohm.
It is constructed from a ceramic cylinder which has a thin coating of carbon to give the resistance. At each end there are copper contacts deposited onto the carbon. The copper is well tarnished after the thing has been languishing an my loft for 30 years or so!

The two smaller ones should be 80 and 52 ohm, but they measure at 60.7 and 37.7 ohm.
How or why they went low in value I do not know, maybe they have absorbed moisture, or they are just cheap junk!.
I am not sure how they are construced, I think they are just carbon cylinders, the inside surface is conductive.


As for using the Cantenna, it will be useful as it is for getting used to tuning the driver and PA of that Tempo One.
I just had a look at a picture of one, it looks like a "badge engineered" Yaesu FT200, a very popular (cheap) transceiver from years ago.
Beware of the PA valves, they are easy to damage by incorrect tuning and matching to the antenna. They are becoming hard to find and hence expensive!

JimB

 

[Nigel Goodwin]

Nigel - I've never used a dummy load before, but every ham I have talked to says I will want one. As I understand, it is hooked up in place of the antenna and allows for tuning of the transmitter without going on the air. Learning how to use it will be my next step, but I figure I need to have it working right if I expect to get any kind of accuracy when I do use it. I got my radio (Tempo One Transceiver) a long time ago, and I have never tried the transmitting side of it. I've sure it is way out of adjustment.

As I've used dummy loads it's to align the output of a homemade transmitter to be (fairly) accurately 50 ohms - you then adjust the aerial to match the transmitter. But HF aerials don't really work like that, 'proper' ones are generally too large, so you use an ATU (Aerial Tuning Unit), which may be internal to the transmitter, to match the transmitter to the (non-50 ohm) aerial.

Presumably, from what JimB said, you may have to align the transmitter when you change bands, and retune the driver and PA stages - THEN retune the output stage to match the aerial?.

 

[stevez]

Andy - a few comments on the dummy load and antennas in general.

1. I'd expect that if the resistor was a constant 77 ohms that it would serve adequately as a load in many cases. My concern is that with something so far off specification - will it simply fail open presenting high resistance. If you were doing laboratory work then precision would be important. So don't let me mislead you - if you had a reliable 77 ohm resistor you'd be ok for many situations.

2. The antenna is what radiates the energy. The transmission line is the line that connects the source of power to the antenna.

3. The transmitter is designed to operate into a 50 ohm resistive load - or something close to it. How close is quite dependent on the vintage, among other things. Tube/valve final rigs tend to have greater tolerance in that their pi or other matching sections can adapt to a wide range of impedances.

4. Ideally, we'd have the antenna look like 50 ohms resistive at the point of connection - and we'd use 50 ohm transmission line - so the antenna matches the transmission line. If that were the case, at the frequencies of interest, then the transmission line would look like 50 ohms to the transmitter.

5. In reality the antenna probably won't look like 50 ohms at the feedpoint. You can do some things to make it looks like 50 ohms - gamma match, stub match or a tuning unit right at the antenna feedpoint. All sorts of things affect feedpoint impedance - best to read a manual. You may also accept the non-50 ohms and connect the antenna to the transmission line.

6. If the antenna isn't 50 ohms at the feedpoint then the point of connection at the transmitter to the transmission line wont look like 50 ohms - it will be something else. At this point you might use a tuner of some kind - usually a variable capacitor or two and possibly an inductor. A tuner at the point of connection to a transmitter will do nothing to tune the antenna. All it does is transform the impedance presented at the transmission line - so that the transmitter sees a load it can tolerate.

7. Your dummy load, once determined to be reliable, can aid you in powering up the transmitter. You can see if it behaves per instructions with appropriate dipping or peaking as you adjust the controls. You can check input current and other things. If you can't get the transmitter to behave in a predictable way into a proper dummy load - don't consider putting it on the air. Failed tubes or gross mistuning/misalignment can result in improper emissions.

8. You can connect your dummy load along with a SWR meter. You should see little or no reflected power on a short run of coax. As a good test of your coax you can put the load at the end - where you'd otherwise put the antenna. You should measure approximately 50 ohms (if the load is 50 ohms) and you ought to see little or no reflected power - in fact with longer coax there should be less reflected power than with shorter as the losses in the coax are a load. If you don't see low reflected power it suggests a failure in the coax or possibly the connectors.

Anyway, it's a good item to have on hand. I'd encourage you to have a dummy load that is more on spec - simply because you can't be sure of what might happen with a 50 ohm resistor that measures 77 ohms.

I've taken some big shortcuts with the explanations - my apologies for that however, proper explanations are available in the many publications that are available.

 

[Russlk]

You can still use the cantenna at 77 ohms, there will be a standing wave ratio (SWR) of 1.5:1 but if you are using a short 50 ohm line it will be of no consequence, and 1.5:1 is not so bad anyway.

 

[Andy1845c]

Thanks for the awsome reply Stevez! I just saved it to my hard drive. I'll have to see if I can find a replacment resistor. I take it you must be a ham radio operator?


JimB - Those look very much like my resistor! The ends of mine are kind of a sulfer yellow though, as opposed to the tarnished copper color. Would it hurt/could it help to take some fine sandpaper or steel wool and clean the ends of mine? Mabey they are corroded, but it dosn't look like they are to the naked eye.