Can Tenna

[George T. Baker]

A couple of thoughts here:

1. RF resistance - which determines SWR as a load - seldom equals DC
resistance.

2. Unless a high-accuracy bridge operating at the desired frequency is
being used, the "measured" resistance may be in error by several if not
many percent.

3. Even the better analyzers - AEA/Tempo CIA-HF, for example - are not
spot on in readings. Less sophisticated units can be pretty far off,
especially at the high end of the HF spectrum.

4. Since the RF resistance of the Cantenna element is probably NOT 50
ohms, the length of any coax between the Cantenna and the measuring
device will alter the measured "load resistance." This effect is
especially noticeable at the high end of HF.

I built my Cantenna  in 1970 and got it filled with PCB-laden transformer
oil at the local power company. Except for a couple of misguided years of
QRO operation with an SB-220, I have always operated at 100 watts or much
less.

Following are the results of measurements made using my CIA-HF connected
through 4 ft of RG-8X:

        Frequency (MHz)         SWR             R (ohms)        X (ohms)

        3.5                     1.05            41.7            0

        7.0                     1.05            41.8            0

        10.1                    1.04            42.2            0

        14.0                    1.04            43.5            0

        18.11                   1.03            43.7            0

        21.0                    1.04            43.4            0

        24.95                   1.04            43.7            0

        28.0                    1.03            43.8            0

My conclusion is that the old greasy can is still a very good dummy load,
at least over the HF range. But, I would caution that simple VOM
resistance measurements are not likely to tell the whole story.

Not "faulty" but certainly not "perfect."

72/73, George        AMA 98452          R/C since 1964
Amateur Radio W5YR, in the 53rd year and it just keeps getting better!
AutoPOWER Systems, Fairview, TX (30 mi NE Dallas)  Collin County
QRP-L QRP-ARCI FISTS NORCAL ZOMBIE ARS 10-X  33.2 N 96.6 W EM13RE


Tom Bowman wrote:
>
> When I built my Cantenna in 1975, I measured the resistance
> after it was assembled and I had poured oil into the can.
> It measured 41 ohms. Since the resistor was coated with oil,
> I decided not to send it back to Heathkit for a replacement.
>
> Again, this was before using the Cantenna.
>
> I suspect there are more Cantennas with faulty elements
> out there.
>
> 73,
>
> Tom, WA3REY

Sponsored by the City of Tempe 

Listserver Submissions:  heath at listserv.tempe.gov
Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath 'name' 'call'"
Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"