Hi Scott

There are many types of Discone antennas available depending on what
frequency range your interested in receiving. Cable is a very
important consideration and often overlooked. When I purchased the
Icom AH-7000 Discone antenna, about twelve years ago, a fifty foot
section of their so called "low loss" cable was included, and
terminated with "N" connectors at both ends. Personally I wouldn't
even give that included cable away to someone I didn't know, as it is
pretty poor quality.

When this particular Discone was installed, the cable I selected was
Belden 9913. This cable has a 9 1/2 AWG solid center conductor, a
solid spiral teflon tube continuously running around the center
conductor which is enclosed in a air dielectric teflon tube bonded with
copper foil on the outside and that is covered with a braided copper
shield and covered with a soft PVC water-proof jacket. There is an "N"
connector with the center pin specifically made for this exact cable.

Since this cable is not as flexible as some installations may require,
"Cable Experts" has a "9913" equivalent cable which is a little more
flexible, it's "CXP1318" and is available in various lengths terminated
with either an "N" or "PL-259" connector.

Believe it or not, the difference in this cable compared to the cable
included with the Discone was significant. This test was made with the
Icom AH-7000 Discone using equal lengths of both cables, fifty feet,
and both cables were terminated with type "N" connectors. Using my
Icom IC-970H multi-mode transceiver, I observed about SEVEN S units in
signal strength at 432Mhz. That's right, SEVEN! I now have a twenty
two foot section of "Andrews Heliax" hardline cable connected to a
"KB6KQ 432Mhz loop antenna". Realizing I'm getting off topic here, the
point I'm attempting to make is the CABLE is a VERY important issue!

May I suggest that before you finalize your anticipated antenna
installation, do some testing with antenna placement, you may find that
it may not be necessary to have it mounted outside in the weather. I'm
located right on the beach and routinely need to remove, clean and
replace my Discone every few months due to corrosion. This next week
I'll be purchasing the AOR DS3000A Discone antenna. This will be
mounted inside with only a six foot section of CXP1318 cable and
dedicated to the Uniden BC796D I purchased just last week. Believe it
or not, it is my very first "scanner".

You may even have an area in the attic which will accommodate your
installation. I say this because when you start installing antennas
outside, sometimes your neighbors suddenly become interested in what
your doing. Something else you may want to consider. My nearest
neighbor is probably less than a mile away, so I do not worry about
this and have various antennas mounted outside only because their also
used for transmitting as well.

If you do not plan on using your Discone for transmitting you may get
away with it mounted inside. Above all, use good cable and if possible,
keep cable lengths to a minimum!

Best regards

george



On 2005-10-29 12:12:53 -0700, Al Klein said:

On Fri, 28 Oct 2005 14:30:45 -0400, Scott said in
rec.radio.scanner:

Could someone really help a newbie out on what i would need to buy to get
this antenna just right for scanning. I'm not afraid to do anything,
but seriously just don't know what all I need, and what i should
expect from this type of setup.

Four things you should be aware of:

1) A discone is a negative gain antenna - that is, it has less gain
than a dipole, which is the standard by which 0 gain is measured.
Since the signal it receives is so weak, the cable is VERY important -
the lower the loss the better, even if it's a fairly short run. DON'T
buy your cable from Radio Shack - they don't sell any low loss cable -
only cable that's lowER loss than the regular cable. Go to this site
http://www.timesmicrowave.com/cgi-bin/calculate.pl, plug in the
highest frequency you're interested in (will you be listening to
850?), the cable type (try a few of the common ones - RG6, RG8, RG58)
and the approximate length of cable, and you'll see how much signal
the cable will lose. Look at the "efficiency". Subtract that from
100 and that's how much signal is being lost.

Don't let anyone tell you that it only matters if you're transmitting.
Do you see any "direction" choice there? The same amount of signal is
lost in both directions. (Actually a little more is lost with a
scanner because the scanner isn't a 50 ohm load to the cable.)

Be prepared to spend at least 50 cents/foot for decent cable and up to
$2/foot for good cable. Putting up a gainless antenna and lossy cable
(like 100 feet of RG58 for 850 MHz - 96% of the signal is lost in the
cable) is just a waste of time and money. You'll probably receive
better with a rubber duck on the scanner.

2) Don't buy hype, buy an antenna.

If a discone is designed properly, and very well made, it will cover a
frequency range of 4:1. (Most real - not on paper - discones are more
like 3:1.) The lowest frequency is that at which the radials are 1/4
wave long. So to receive VHF-lo well they have to be about 6 feet
long. The highest frequency this antenna will be any good at will be
about 120 MHz.

The typical discone with 3 foot radials will cover from about 75-225
MHz.

25-1300 MHz is advertising copy, not measured performance. If anyone
can get a discone (without a whip sticking up on top - just a pure
discone - if you want whips, buy a Scantenna, or something like it) to
cover a 50:1 frequency range (25-1300), you're going to hear him named
a Nobel prize winner in physics. And science will have to be
rethought.

ANYTHING that's not an insulator will receive signals. The question
is whether that thing, at the end of some reasonable length of
affordable cable will receive any more signal than an antenna plugged
right into the scanner. Why put up an antenna that gives you 6db gain
over the antenna on the scanner and connect it with cable that gives
you 9db loss? You just REDUCED the signal you're going to hear by 50%
(3db). Sure the antenna has 6db gain over the one on the scanner -
but it's not going to do you any good unless the cable loss is less
than 6db.

3) Wherever you put the antenna, and no matter how high you put it,
figure out where it will fall if the mount breaks away at the base. It
should fall within the confines of your property - for a few of
reasons.

1) You don't want a neighbor getting hurt if it falls.
2) You WILL be considered legally negligent if you mount it so that it
can fall outside your property line.
3) Your insurance won't cover the judgment against you - you'll pay it
all out of your pocket.

But don't mount an antenna on your chimney. Aside from the fact that
a chimney isn't made to withstand the torque produced when the wind
hits the antenna, part of what goes up the flue is sulphur and, when
it comes into contact with water (like from air with any humidity at
all), it forms sulphuric acid. Do you really want your antenna bathed
in that for months at a time every year?

Add last, but by no means least, if you're going to put up an outside
antenna, PLEASE put in a good grounding system, ground the mast to it
and use a static discharge device on the cable. The ground system
should tie ALL the grounds in the house to one point - telephone,
electric, antenna. And a decent ground (not a good one, a decent one)
is AT LEAST 4 10 foot ground rods spaced in a rectangle at least 10
feet on a side, with wire going from 1 to the next, but NOT forming a
loop, with all your grounds connected at one point along that run -
any one point, preferably near the middle.

I'm not making this up - read the National Electric Code on grounding,
or ask an electrician. This is important - people are killed every
year by bad antenna installations. Not many - but if you're one of
them, it doesn't matter how many there are.

With all that in mind, this is a hobby, so enjoy it.