No antenna will work across the full 75/80 meter band with low enough VSWR to
accommodate all radios. REASON: the bandwidth range +/- 13.3% centered on 3750
KHz to the band edges is excessive. If you cut the antenna for 3.9 MHz it most
likely will not load up well below 3.7 MHz. A tuner is required!! PERIOD.

For comparison, the bandwidth range on 20 meters centered on 14.175 is only +/- 2.4%

Don't jump on condemning the antenna too soon. Do some more research and ask
more questions regarding 75/80 meters broadband antennas and what "broadband" means.

/s/ DD, W1MCE

wrote:

Buyer Beware
I have read all the reviews and I am wondering if they are about the
same antenna I purchased. I bought the Cobra junior antenna and
followed the directions for installation. The antenna is 50ft in the
air in a flat top configuration. When I hooked it up to my radio, it
would not tune on segments of certain bands, most noticeably 75/80. I
called Joe to get some help and was told that my radio, which has a
built in tuner would not accept more than a certain level of SWR and
was told to trim the twinlead back 19 feet. I did and while some of the
problem was solved, I continued to have problems on 75/80. I emailed
Joe to ask for further assistance and was told that the next step was
to buy a tuner extender (price $60). Not the answer I wanted to hear.
If in fact this kind of problem exists with radios that have built in
tuners, prospective buyers should know that in advance. For the cost of
the antenna and now the tuner extender, I could have bought a better
antenna, which is what I intend to do. I am not pleased with this
antenna and would not recommend it especially to someone who has a
radio with a built in tuner and intends to use it. Additionaly, the
performance on 40 seems to be very good. It seems to under perform on
20. Can't tell on 75/80 because I can't tune the band.

KC2GWK

Dave wrote:
No antenna will work across the full 75/80 meter band with low enough
VSWR to accommodate all radios. REASON: the bandwidth range +/- 13.3%
centered on 3750 KHz to the band edges is excessive. If you cut the
antenna for 3.9 MHz it most likely will not load up well below 3.7 MHz.
A tuner is required!! PERIOD.

For comparison, the bandwidth range on 20 meters centered on 14.175 is
only +/- 2.4%

Don't jump on condemning the antenna too soon. Do some more research and
ask more questions regarding 75/80 meters broadband antennas and what
"broadband" means.


Be kind folks! You're all correct, but don't bash the guy's head in.


75 and 80 meters is indeed quite a range to cover, especially when it
is actually 85 to 75 meters in wl.

I'd buy a tuner that will handle open wire feed, lose the 4:1 balun,
and get on the air.

- 73 de Mike KB3EIA -

On Thu, 08 Jun 2006 20:33:12 -0400, Mike Coslo
wrote:


I'd buy a tuner that will handle open wire feed, lose the 4:1 balun,
and get on the air.

Mike, did you notice "Mystery's" comment re spending more money on a
"tuner extender": "Not the answer I wanted to hear". She/he may be
even less interested in the spend on the tuner you describe.

I agree with you that antenna looks worth a try / perseverance, but it
will probably need a wider range tuner than commonly fitted internal
to transceivers unless you experiment (what's that!) to find a
feedline length that the rig/runer accommodates on all bands, or
switch feedline lengths.

Owen
--

On Fri, 09 Jun 2006 00:59:53 GMT, Owen Duffy wrote:

On Thu, 08 Jun 2006 20:33:12 -0400, Mike Coslo
wrote:


I'd buy a tuner that will handle open wire feed, lose the 4:1 balun,
and get on the air.

Mike, did you notice "Mystery's" comment re spending more money on a
"tuner extender": "Not the answer I wanted to hear". She/he may be
even less interested in the spend on the tuner you describe.

I agree with you that antenna looks worth a try / perseverance, but it
will probably need a wider range tuner than commonly fitted internal
to transceivers unless you experiment (what's that!) to find a
feedline length that the rig/runer accommodates on all bands, or
switch feedline lengths.

Owen


There really isn't a lot of information on the antenna on the page I
saw: http://www.k1jek.com/. Many antennas have a spectrum of
frequencies and their associated SWR. Knowing my internal tuner can
only handle 8:1 but the antenna shows places I am interested in using
with 12:1, would help me in my decision to choose the antenna or
another. the OP didn't have that information. I know that internal
antenna tuners are basically 'fine-tuners' and are very limited
compared to external tuners. (at least Kenwood is clear about that in
their instruction guides. ) I am not pointing a finger at either
party, Mystery for not posting the information or some warning nor the
OP for not asking. The Mystery page isn't very Corporate looking so
one would have to assume some give and take in the purchase by both
parties.

Is there any reason the antenna can't be returned for a refund? If
not, can it be sold to someone else who is interested?

This is one of life's lessons learned.


I just looked at the review of the antenna from eHam.com:
===============
WA1BSB Rating: 5/5 Apr 24, 2006 14:29

Homemade Ultrilite Senior Time owned: 0 to 3 months

Hi I went and made a homemade Ultrilite I too 3 conductor tv rotor
cable 70 ft each side and mounted it to a modified 450 ohm ladderloc
at the center I then ran about 50 ft of ladder line down to a 4.1
balun I know the plan calls for 100 ft but I just dint have the spot
to coil it up The antenna is in a flat top configuration up between 2
trees about 80 ft up from the balun I ran about 70 ft of RG8X 50 ohm
to my 746 Pro it tunes perfect on all bands 10-160 using the radios
tuner 1.1 swr Anyhow it works with good signal reports so far on 40
and 20 meter Im looking forward to trying in 75 and 160 Anyhow bottom
line whether you make one or buy it its quite the antenna I glad I got
one 73s
==================

There are many other reviews about the antennas, both jr and sr.

Maybe there is a better configuration that will work, or he may need
the tuner or tuner extender.

Good luck

Buck


--
73 for now
Buck
N4PGW

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?

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?

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).

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.

Owen
--

Owen Duffy wrote:
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.

Here's what my IC-756PRO manual says about its internal
tuner: "*Matching impedance range: HF bands, 16.7 to 150
ohms unbalanced (Less than VSWR 3:1)"
--
73, Cecil http://www.qsl.net/w5dxp

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

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


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.

Well, the antenna system looks promising on some bands, but not
others. Draft article is at http://www.vk1od.net/cobra , constructive
comments appreciated.

The article includes a link to a graph of the modelled system losses
for the Cobra Senior, but it looks pretty uninteresting with ~9dB of
system loss at 160m... there is no shortage of poor performing
antennas on 160m.

Owen
--

Owen Duffy wrote:
On Thu, 08 Jun 2006 20:33:12 -0400, Mike Coslo
wrote:


I'd buy a tuner that will handle open wire feed, lose the 4:1 balun,
and get on the air.

Mike, did you notice "Mystery's" comment re spending more money on a
"tuner extender": "Not the answer I wanted to hear". She/he may be
even less interested in the spend on the tuner you describe.

Of course.

But sometimes the unwanted answer is still the correct or best one.....



I agree with you that antenna looks worth a try / perseverance, but it
will probably need a wider range tuner than commonly fitted internal
to transceivers unless you experiment (what's that!) to find a
feedline length that the rig/runer accommodates on all bands, or
switch feedline lengths.


Sounds like time for Cecil to join in. I know his system uses
switchable lengths of twinlead, but perhaps he might have something to
offer here in a one length feedline system.

- 73 de Mike KB3EIA -

Michael Coslo wrote:
Sounds like time for Cecil to join in. I know his system uses
switchable lengths of twinlead, but perhaps he might have something to
offer here in a one length feedline system.

Anyone have an EZNEC model of this antenna?
--
73, Cecil http://www.qsl.net/w5dxp