75 ohm CABLE coax usable w/dipoles?
Snorky
Would this type of coax work okay for a ~100w rig such as the 530S and
dipole(s) and/or slopers? I have access to alot of this type of cable
for free.
As I recall it is 75 ohms, and dipole/slopers are about 75 ohms as
well, but I am not sure it would work okay.
tnx - 73 - Ed - KF6YKM - Southern California
Phil Gagner
For VHF, see http://members.mint.net/n1bug/tech/catvline.html.
For HF, the impedance of, say, a 40-meter dipole mounted where most people
put it (about 1/4 wave or 34 feet above ground) is between 72 and 75 ohms,
so the 75 ohm cable will match it nicely.
You will need to worry about the match at the output of your rig (which
probably expects 50 ohms). If you don't do this, you'll get an SWR of
something like 2 right off the bat. You could, of course, use an antenna
tuner or transmatch. The loss should be small, especially because you'll be
using a short piece of 50 ohm coax between the transmatch and the
transmitter.
One worry would be the physical quality of the cable. Obviously, you have to
put on new connectors, and do it right. Use coax with a PVC coating
(assuming your antenna is outside). Also try to get non-hydroscopic foam,
which is moisture resistant. Otherwise you won't be running at anything like
75 ohms. This stuff looks and feels like hard foam, glued to the copper
center conductor. If the coax is for indoor use, then it won't work long for
well outdors. The heavy stuff that the cable TV companies use outdoors is
generally pretty good stuff, and the thin and more flexible indoor stuff
generally isn't.
The next worry is feed line loss, and again depends on the quality of the
coax. Cheap thin coax has much higher loss than quality heavy coax. The
longer your feed line, the more loss you get. For short lines, it generally
doesn't really matter. If you're running 50 feet or more, then it does.
The rule is:
Feed line loss = connector loss + cable loss + mismatch loss
The loss you get at the connectors depends very much on how well you install
them, and how good they are. The feed line loss depends on the quality of
the coax and how long it is. The mismatch depends on whether you use a
transmatch or a transformer, or you could probably even use something like a
stub.
73's Phil Gagner - KE4PDP
Snorky <poof...@eudoramail.com> wrote in message
news:<39529b41...@news.gte.net>...
w5lz
Snorky,
The short'n dirty answer is, Yes, it will certainly work.
Depending on how the antenna is erected, surroundings, etc,
you probably couldn't tell the difference between the 75 ohm
feed line, and a 50 ohm feed line. Besides, if it's free,
what are you going to loose if it doesn't?
'Doc
Fractenna
>
>Would this type of coax work okay for a ~100w rig such as the 530S and
>dipole(s) and/or slopers? I have access to alot of this type of cable
>for free.
>
>As I recall it is 75 ohms, and dipole/slopers are about 75 ohms as
>well, but I am not sure it would work okay.
>
>tnx - 73 - Ed - KF6YKM - Southern California
>
Hi Ed--
A 1/2 wave dipole has a feedpoint of order 75 ohms, but it decreases when
placed less than about 1 wave above ground.
You can use it, but there will be a mismatch from the rig to the coax, which is
a rather minor insertion loss.
I would suggest that you choke the dipole at the feepoint with six turns of
coax (about 6 inches in diameter) or with some ferrite chokes (I assume this is
an HF dipole).
73
Chip N1IR
Tom W8JI
Chip's answer is pretty good, but let me clarify a few points.
On 23 Jun 2000 01:03:16 GMT, frac...@aol.com (Fractenna) wrote:
>A 1/2 wave dipole has a feedpoint of order 75 ohms, but it decreases when
>placed less than about 1 wave above ground.
Actually the impedance goes in a "spiral", at very low heights over
normal soil the impedance can be higher than 75 ohms. As height is
increased, the impedance drops. Eventually it increases again.
I'm not sure how low it actually can go over various soils, but it
certainly go below 50 ohms at modest heights.
That may give you a problem, with certain length feedlines. If the
feedline approaches an odd-multiple of 1/4 wl, it will tranform the
impedance to a much higher value. That means for various lengths of
feedlines the impedance can vary from 50 ohms or so up to 100 ohms or
more.
Your radio, without a tuner, might not "like " the impedance it sees.
You could use a tuner, or in some case you might cut the feedline
until you find a length the rig likes.
Some rigs favor a lower impedance than 50 ohms, some favor an error on
the high side (my IC-751A is that way). Some won't tolerate much error
at all (my FT-1000 is that way).
So you can run into trouble, but you also can probably work around the
problem.
>You can use it, but there will be a mismatch from the rig to the coax, which is
>a rather minor insertion loss.
Insertion loss is the least of your worries at HF. The real problem is
the impedance mismatch, which can cause your rig to "shut-down". A
secondary problem is IMD, when the PA is mismatched the final can
become less linear causing splatter.
In any event, I have used CATV cable for years on my transmitting
antenna. RG-6 will handle anything you can throw at it on lower bands,
despite what the ratings and nay-sayers will tell you.
73 Tom
Fractenna
>
>A 1/2 wave dipole has a feedpoint of order 75 ohms, but it decreases when
>placed less than about 1 wave above ground.
>
>is
>a rather minor insertion loss.
>
>coax (about 6 inches in diameter) or with some ferrite chokes (I assume this
>is
>an HF dipole).
>
>73
>Chip N1IR
My apologies for the typo. I cut my fingers on some coral last week and
occasionally I skip a key key:-)
The above should read:
"... but it decreases when
placed less than about 1/3 wave above ground."
In fact, the impedance actually increases to over 85 ohms at about 1/2 wave, if
memory serves. Then it go back down to about 75 ohms when you approach 1 wave.
Perhaps someone can dig this out of a book as my memory is not always precise.
The canonical "50 ohm dipole" is a low one indeed.
73
Chip N1IR
gla...@glass2.lexington.ibm.com
>>
>> Would this type of coax work okay for a ~100w rig such as the 530S and
>> dipole(s) and/or slopers? I have access to alot of this type of cable
>> for free.
>>
>> As I recall it is 75 ohms, and dipole/slopers are about 75 ohms as
>> well, but I am not sure it would work okay.
>>
>> tnx - 73 - Ed - KF6YKM - Southern California
No, it obviously won't work, and the original poster, in order to
relieve the temptation to try it, should gather up all of his 75 ohm
coax and ship it to me.
Hmm, didn't buy that, eh? Oh, well, it was worth a try. :*)
Actually, it will work. I've done it. However, there are a couple
of concerns. The first concern is whether the radio is capable of
being matched to the cable. Some of the older radios, especially
the tube type radios, did have enough tuning ability to match into
75 ohm loads. An antenna tuner may also be used.
Next, some 75 ohm coax is pure junk. There's some good stuff out
there, but there's some awful stuff, too. I think that some of it,
especially some of the cheaper RG-59 cable, is made for closed
circuit television cameras, and the quality on some of this is
noticably lacking. One sample I've obtained only has a few wires in
the braid, intending for an aluminum foil layer to provide most of
the shielding. Putting anything other than a crimp-on connnector on
this is a complete exercise in futility. (The good news is that the
center conductor is a copper coated steel wire, so it makes a
passable long wire antenna, even if it is a bit heavy.).
I've also obtained some quality 75 ohm, RG-59 cable (Belden, I
think) that was a real pleasure to work with. I've also used some
RG-11 cable that wasn't too bad, although that was quite a while
ago.
Connectors may be a bit of a problem. RG-59 isn't too hard to get
"UHF" style PL-259 connectors for, but you do have to remember to
get the reducers sized for it. Fortunately, the reducers are
readily available since the same reducer is used for the RG-8X
"Mini-8" cable. BNC or N connectors may be a different matter, as
may be connectors for other cables.
In any case, the most important aspect of the cable is the loss
factor. If you have any question about the quality of the cable,
scrap it and get some good stuff (e.g., Belden 9913, or equivalent).
Dave
P.S. Standard Disclaimer: I work for them, but I don't speak for
them.
Jynx
> ...,orr in some case you might cut the feedline
>until you find a length the rig likes.
This would only work for a specific frequency (or across a very narrow
band of frequencies) and/or frequencies harmonically related.
This is the 'trick' I employ for use of 1/2" cable TV hardline on
6 Meters and 3/4" cable TV hardline on 2 Meters.
On 6M I do not expect to operate much outside the 50.090 to
50.400 mcs range -- and it seems to work quite nicely that-a-way.
On 2M my operating range spans about 144.100 to 144.250 -- and,
it seems to work quite nicely that-a-way there, too.
The most effective feedline, of course, is one connected to your
transmitter at the one end -- while the other end is connected
to an installed antenna --- versus the one that is coiled up in
your garage, while you peck away at the keyboard -- looking for
assurances that what you are attempting
is The Best Of All Possible Solutions.
It never is -- The Best Of All Possible Solutions, that is.
That's why we call it Amateur Radio, and why we learn things.
gl es 73,
Jonesy W3DHJ
--
Marvin L Jones | jonz | W3DHJ | OS/2
Gunnison, Colorado | @ | Jonesy | linux __
7,703' -- 2,345m | frontier.net | DM68mn SK
bsnvu...@gmail.com
> Hi Guys!
>
> Would this type of coax work okay for a ~100w rig such as the 530S and
> dipole(s) and/or slopers? I have access to alot of this type of cable
> for free.
>
> As I recall it is 75 ohms, and dipole/slopers are about 75 ohms as
> well, but I am not sure it would work okay.
>
> tnx - 73 - Ed - KF6YKM - Southern California
Allodoxaphobia
The conclusion is that yet another google grooper
has replied to a NINETEEN-YEAR OLD posting.