On Fri, 09 Jun 2006 05:13:04 GMT, Owen Duffy wrote:

On Fri, 09 Jun 2006 00:05:47 -0400, Buck wrote:

frequencies and their associated SWR. Knowing my internal tuner can
only handle 8:1 but the antenna shows places I am interested in using

Buck, how do you know that your tuner can handle (presumably SWR up
to) 8:1? Is it a commonly published specification for transceiver
internal tuners or external tuners?

In the manuals or specifications of most radios I have looked at with
internal tuners, they say somewhere what the tuner can match. The
Kenwood TS440SAT had in its instructions somewhere that it could match
antennas with SWR up to 10:1 (which seems to be typical of most
internal tuners on radios.) I have read the specs of many rigs
including Yaesu, Kenwood and Icom. I can't say which have the specs
and which don't, but I remember reading the specs many times for many
radios.



The capability of a tuner is much more complex (pardon the pun), but
could be mapped for each band to a Smith Chart as an area of input Z
that could be transformed to 50 ohms, and then one could plot loss
contours to show the acceptable range. What would you do with the
information?

I would read the radio specs, check my antenna and if the antenna
matched, it matched, if not, I would use an external tuner, adjust the
antenna, replace the antenna or repair the radio if necessary.


Whilst it may be appropriate to specify the tolerance of Z on a
nominal 50 ohms antenna with a maximum VSWR in a given frequency range
(eg as often done for VHF and UHF antennas), the Cobra does not
pretend to present any particular impedance on any particular
frequency (remember that user is permitted to use any length of open
wire feed operating at high VSWR, so the Z at the tx end of the balun
cannot be specified at any frequency).

I saw NO specifications on the antenna related to resonance when I
looked (the link is posted on an earlier posting.) This leads me to
believe that the antenna is made by a hobbyist rather than a
professional corporation. This isn't necessarily bad, but it would
mean I would look into it further before buying. Reading the reviews
in eHam, etc, which I believe pointed to a good antenna not addressing
your specific problem. However, having as much information as I have,
I would have done what the first reviewer on eHam did. I would build
my own and test it.

The Cobra Junior, a 72' long linear loaded dipole, looks interesting
in terms of a short antenna for low HF bands with reasonably low
losses. I have run some preliminary NEC models and analysed the feed
system loss with 25m of W551 ladder line, they look promising. I will
build a complete model over HF along the lines of the ones that
underly my article on the G5RV feed system at
http://www.vk1od.net/G5RV/index.htm . More when it is done in a day or
three.

I will be glad to look at that. I hope it helps the OP.


Owen

Thanks,

When looking at antennas, I first look at all the information
available from the manufacturer, then I look for reviews such as at
eham, and I might bring it up for question here and at similar forums.

The antenna in question has no specifications or promises, per se,
except for the claim that one can tune it to all the bands. This
claim is reinforced by the reviews on eHam.com. As I said earlier, I
wouldn't buy it, I would build it. I like doing that. If I were
to purchase it and had an internal tuner in the rig, I would ask the
maker or seller of the antenna if it is expected to work on my
particular radio. He might inform me that the antenna might not work
on some internal tuners and I would be better informed before I
purchase.

Of course, the OP apparently doesn't have this information, or didn't
read it about his particular rig. Then again, there are very many
external factors that could affect the antenna.

I recently helped someone connect an 80 meter antenna to a TS-440 SAT
which has an internal tuner. When we tested it in one position, it
worked perfectly. However, when we added 6 feet of coax to move the
radio and it suddenly tuned everywhere in the mars 4 mhz band except
the area around which he meets in his net. We had to trim the antenna
to make it match. Strange, but true.

I didn't analyze the whys and wherefors, but we did get the man
operating successfully, which was the purpose of our visit.

Well, I have to get ready for that four letter word forbidden on local
repeaters here.

73 for now.
buck

--
73 for now
Buck
N4PGW