Brian and Gary...good stuff. Now understanding the loss of power, it
would make sense to get an amp (Idon't think it would be legal) or have a
second line run to the spreader on the mizzen.That would drop the run down
to 30' and a corresponding increase in power.
Thanks for you help
Bruce
"Gary Schafer" wrote in message
...
This is the same Larry that says that he has a 150 watt VHF amplifier
to switch in the antenna line if he can't be heard.
Yet he recommends using lossy rg58 cable. Old surplus stuff at that!
When cable ages it does not always do so gracefully. The dielectric
breaks down and the losses can be considerably higher than new cable.

For the amount of work involved and the minimal cost of 100 feet of
RG8 type cable, use the best NEW cable.
RG8 with polyethylene insulation is the way to go unless you have a
size restraint. And forget the aluminum wrap shield stuff. You are
asking for trouble in the marine environment with aluminum.
You do not need "high shielding" for a VHF radio.
Foam insulation is not a good idea on a boat though. The center does
migrate at bends. It is also very difficult to install connectors on
foam cable. The heat from the soldering iron melts the insulation
quickly and lets the center wire move to the side. Sometimes even
shorting it.

Regards
Gary


On Sat, 17 Jan 2004 18:53:08 GMT, Brian Whatcott
wrote:

I recommend checking with suppliers' specifications for cable
construction materials: solid or stranded? solid or foam insulation?
%coverage of shield? Tape wrap? Non-acid outer sheath? before
blindly implementing Larry's recommendation below on choosing
cables. Yes: let's call it a reality check.

Brian W



On Sat, 17 Jan 2004 05:54:58 GMT, (Larry W4CSC) wrote:

... 55' mainmast has a Shakespeare 1/2 wave VHF antenna on top
with about 75' of RG-58A/U military surplus cable///
If I had to start from scratch, and couldn't buy a 500' roll of
surplus RG-58A/U for $10 at my local thrift shop...

I recommend using VERY flexible, and reliable,
Polyethelene-cored, finely stranded center conductor RG-58A/U (not
RG-58/U which has a solid center conductor) for your purpose. All
those fancy loss charts mean nothing when the fancy foam cables are
pulled through a sharp turn, somewhere. Hell, you can wind RG-58A/U
in a hangman's noose and it'll still work great! REALITY CHECK!!

On Sun, 18 Jan 2004 06:15:40 -0500, "Bruce"
wrote:

Brian and Gary...good stuff. Now understanding the loss of power, it
would make sense to get an amp (Idon't think it would be legal) or have a
second line run to the spreader on the mizzen.That would drop the run down
to 30' and a corresponding increase in power.
Thanks for you help
Bruce

The amp IS illegal under normal use. However, most don't realize that
in an emergency situation where life is endangered, all radio laws are
null and void and any power is fine if it saves lives.



Larry W4CSC

As has been pointed out, the cable loss (within reason), is a somewhat moot point.

Given an antenna height of X, it is not hard to calculate the maximum expected range, plus some for
refraction, then also given the typical receiver sensitivity, (assume 10 microvolts for a
not-so-good receiver), and antenna gain, you'll find that the power required at the antenna is not
much. When the boat's rock and roll effect on the antenna pattern is considered, the power
requirements of course must go up. As a side note: low gain antennas are better in this respect,
assuming the low gain is due to an wider vertical beam-width, and not just poor design.

But in my estimation, the real key to reliable marine VHFcommunications is in the connectors.

When a young lad I spent 5 years maintaining antennas (all frequencies) in a coastal environment,
(actually, within 200 yards of the beach) and can assure you that an unprotected/exposed connector
in salt air can bring an antenna to its knees in 3 to 6 months.

Norm

As has been pointed out, the cable loss (within reason), is a somewhat moot point.

Given an antenna height of X, it is not hard to calculate the maximum expected range, plus some for
refraction, then also given the typical receiver sensitivity, (assume 10 microvolts for a
not-so-good receiver), and antenna gain, you'll find that the power required at the antenna is not
much. When the boat's rock and roll effect on the antenna pattern is considered, the power
requirements of course must go up. As a side note: low gain antennas are better in this respect,
assuming the low gain is due to an wider vertical beam-width, and not just poor design.

But in my estimation, the real key to reliable marine VHFcommunications is in the connectors.

When a young lad I spent 5 years maintaining antennas (all frequencies) in a coastal environment,
(actually, within 200 yards of the beach) and can assure you that an unprotected/exposed connector
in salt air can bring an antenna to its knees in 3 to 6 months.

Norm

On Sun, 18 Jan 2004 06:15:40 -0500, "Bruce"
wrote:

Brian and Gary...good stuff. Now understanding the loss of power, it
would make sense to get an amp (Idon't think it would be legal) or have a
second line run to the spreader on the mizzen.That would drop the run down
to 30' and a corresponding increase in power.
Thanks for you help
Bruce

The amp IS illegal under normal use. However, most don't realize that
in an emergency situation where life is endangered, all radio laws are
null and void and any power is fine if it saves lives.



Larry W4CSC

To those that say a few db extra cable loss does not make any
difference consider the following:

Every time you double the height of your VHF antenna you gain 6 db in
signal strength.

6 db signal strength increase at 30 miles accounts for about 3 extra
miles in range.

The amount of loss you have in the cable or because of low antenna
height effects the receiver the same as it does the transmitter.

With an extra 6 db of cable loss you will hear 3 miles less at 30
miles.

When you add antenna height you pick up signal gain from added height
but also increase cable loss due to longer antenna cable length.

There will be a point with a given type of cable that when you double
the height to gain another 6 db you will also lose 6 db in cable loss.
As an example with RG58 at 6 db loss per 100 feet, if you were at 100
feet and went to 200 feet antenna height you would pick up 6 db from
height but would also loose 6 db in additional cable loss.
If you continued to use RG58 it would do no good to increase the
antenna height at that point.

At lower, more practical heights, that are associated with boats the
same problem exists but to a lesser extent. Because you have not
reached the gain / loss (height / cable loss) at lower heights you
still have a gain when you increase antenna height.
But if you use better cable you can realize more of the height
advantage than with higher loss cable.

Remember, every little few db of gain or loss helps. They all add up.

You could reduce your power a few watts and loose a couple of db. You
could use higher loss cable and loose a couple of db. You could mount
your antenna a little lower and loose a few more db. Pretty soon you
have some significant loss. Will you notice the difference in signal
strength when signals are good, no. Can you still talk, sure. Can you
talk as far, no.

To my way of thinking why leave a few db of signal strength laying on
the deck when all you have to do is use a little better cable. That is
probably the cheapest way of all in gaining a few db of signal
strength.

Regards
Gary


On Sun, 18 Jan 2004 06:15:40 -0500, "Bruce"
wrote:

Brian and Gary...good stuff. Now understanding the loss of power, it
would make sense to get an amp (Idon't think it would be legal) or have a
second line run to the spreader on the mizzen.That would drop the run down
to 30' and a corresponding increase in power.
Thanks for you help
Bruce
"Gary Schafer" wrote in message
.. .
This is the same Larry that says that he has a 150 watt VHF amplifier
to switch in the antenna line if he can't be heard.
Yet he recommends using lossy rg58 cable. Old surplus stuff at that!
When cable ages it does not always do so gracefully. The dielectric
breaks down and the losses can be considerably higher than new cable.

For the amount of work involved and the minimal cost of 100 feet of
RG8 type cable, use the best NEW cable.
RG8 with polyethylene insulation is the way to go unless you have a
size restraint. And forget the aluminum wrap shield stuff. You are
asking for trouble in the marine environment with aluminum.
You do not need "high shielding" for a VHF radio.
Foam insulation is not a good idea on a boat though. The center does
migrate at bends. It is also very difficult to install connectors on
foam cable. The heat from the soldering iron melts the insulation
quickly and lets the center wire move to the side. Sometimes even
shorting it.

Regards
Gary


On Sat, 17 Jan 2004 18:53:08 GMT, Brian Whatcott
wrote:

I recommend checking with suppliers' specifications for cable
construction materials: solid or stranded? solid or foam insulation?
%coverage of shield? Tape wrap? Non-acid outer sheath? before
blindly implementing Larry's recommendation below on choosing
cables. Yes: let's call it a reality check.

Brian W



On Sat, 17 Jan 2004 05:54:58 GMT, (Larry W4CSC) wrote:

... 55' mainmast has a Shakespeare 1/2 wave VHF antenna on top
with about 75' of RG-58A/U military surplus cable///
If I had to start from scratch, and couldn't buy a 500' roll of
surplus RG-58A/U for $10 at my local thrift shop...

I recommend using VERY flexible, and reliable,
Polyethelene-cored, finely stranded center conductor RG-58A/U (not
RG-58/U which has a solid center conductor) for your purpose. All
those fancy loss charts mean nothing when the fancy foam cables are
pulled through a sharp turn, somewhere. Hell, you can wind RG-58A/U
in a hangman's noose and it'll still work great! REALITY CHECK!!

On Sun, 18 Jan 2004 23:46:55 GMT, Gary Schafer wrote:

To those that say a few db extra cable loss does not make any
difference consider the following:

Every time you double the height of your VHF antenna you gain 6 db in
signal strength.

Never heard this...any sources I can refer to? None of my books suggest this.

If true...
5 to 10 feet = +6 dB
10 to 20 feet = 12dB
20 to 40 feet = 18dB
40 to 80 feet = 24dB
80 to 160 feet = 30dB, which is 1000 times the power supplied at the transmitter end of the cable.

6 db signal strength increase at 30 miles accounts for about 3 extra
miles in range.

Can you please show the calculations? Or a pointer?
Thanks,
Norm

On Sun, 18 Jan 2004 16:44:18 -0800, wrote:

Every time you double the height of your VHF antenna you gain 6 db in
signal strength

On Sun, 18 Jan 2004 23:46:55 GMT, Gary Schafer wrote:

Never heard this...any sources I can refer to? None of my books suggest this.
///
6 db signal strength increase at 30 miles accounts for about 3 extra
miles in range.

Can you please show the calculations? Or a pointer?
Thanks,
Norm

This is what he had in mind:
If you increase an antenna's height from 40 feet to 80 feet you
increase its line of sight range to the horizon by 3 or 4 miles
( root 2)

And, if you are in free space and are power limited, then doubling the
power transmitted will increase the range by root 2 using the
ordinary inverse square law of surface area [ = power density]
versus radius from a point

So, the argument continues....if the power limited range were 9 miles
and you doubled the power, the power limited range would increase to
12 miles about.
OR
if you doubled the antenna height, the line of sight to the horizon
would increase by about the same amount.
SO
you break even if you lose three dB for an extra 40 feet of height,
and you win if you use less lossy cable than that.

Now in fact, in some cases, VHF communications is not power limited,
but line of sight limited, and it takes MORE than twice the power to
reach an extra 1.4 range or three miles "round the corner" [i.e.
below the horizon]

But using this straight-forward model, you are led towards using
cable which loses less than 3dB per 40 feet or about 7 dB/100 ft.
because that is the breakeven point.

Take home message at VHF:
try to place the antenna as high as possible
and don't kill yourself worrying about cable losses, unless money is
no object. But on a tall boat, better to avoid RG58 if posssible, if
it loses 6 or 7dB per 100 feet.....

Brian W

On Mon, 19 Jan 2004 03:35:58 GMT, Brian Whatcott
wrote:

On Sun, 18 Jan 2004 16:44:18 -0800, wrote:

Every time you double the height of your VHF antenna you gain 6 db in
signal strength

On Sun, 18 Jan 2004 23:46:55 GMT, Gary Schafer wrote:

Never heard this...any sources I can refer to? None of my books suggest this.
///
6 db signal strength increase at 30 miles accounts for about 3 extra
miles in range.

Can you please show the calculations? Or a pointer?
Thanks,
Norm

This is what he had in mind:
If you increase an antenna's height from 40 feet to 80 feet you
increase its line of sight range to the horizon by 3 or 4 miles
( root 2)

And, if you are in free space and are power limited, then doubling the
power transmitted will increase the range by root 2 using the
ordinary inverse square law of surface area [ = power density]
versus radius from a point

So, the argument continues....if the power limited range were 9 miles
and you doubled the power, the power limited range would increase to
12 miles about.
OR
if you doubled the antenna height, the line of sight to the horizon
would increase by about the same amount.
SO
you break even if you lose three dB for an extra 40 feet of height,
and you win if you use less lossy cable than that.

Now in fact, in some cases, VHF communications is not power limited,
but line of sight limited, and it takes MORE than twice the power to
reach an extra 1.4 range or three miles "round the corner" [i.e.
below the horizon]

But using this straight-forward model, you are led towards using
cable which loses less than 3dB per 40 feet or about 7 dB/100 ft.
because that is the breakeven point.

Take home message at VHF:
try to place the antenna as high as possible
and don't kill yourself worrying about cable losses, unless money is
no object. But on a tall boat, better to avoid RG58 if posssible, if
it loses 6 or 7dB per 100 feet.....

Brian W


It is not a linear function though. The amount of additional range
that you get is in the area of 10% to 30% depending on the heights
that you start at.

True line of sight loss (free space) is equal to 6 db every time you
double the range.

When figuring maximum range when changing antenna heights, refraction
of the signal is involved. The above formula also comes into play
slightly too. But it works out to around 6 db of signal increase at
the maximum range when ever you double the antenna height.


Doubling the power and doubling the antenna height do not give the
same results. Doubling the power results in an increase of 3 db in
signal strength. Doubling antenna height results in a 6 db increase in
signal strength. So you would have to increase the power by 4 times to
equal a doubling of antenna height.

Below is a link to a line of sight calculator.
http://www.vwlowen.demon.co.uk/java/horizon.htm


Here is a path loss calculator. It uses 20 miles as a default for the
range.
You can put in some heights and note the amount of path loss in DB
that you get. Then double the height of one of the antennas and note
that the path loss you get decreases by 6 db.


http://www.decibelproducts.com/Calculators/qegli.htm


The "break even point" in cable loss is also a non linear function.
With a moderate loss cable it does not "catch up" to the height
increase until you have reached a couple of hundred feet. With a high
loss cable it can "catch up" at a much lower height. Giving you a
negative return as you go higher with the antenna.

But the point is that the worse the cable loss the more percentage of
power and range you are giving away the higher you go.

Regards
Gary

On Mon, 19 Jan 2004 19:06:06 GMT, Gary Schafer
wrote:
....
Here is a path loss calculator. It uses 20 miles as a default for the
range. You can put in some heights and note the amount of
path loss in DB that you get.
Then double the height of one of the antennas and note
that the path loss you get decreases by 6 db.

http://www.decibelproducts.com/Calculators/qegli.htm

Regards
Gary

This caculator gives the following caution:

"Note: The Egli model is based on data measured over radials mainly in
the eastern US seaboard and central plains states. It gives an overall
propagation loss over gently rolling terrain with average hill heights
of approximately 50 ft. "

You may have noticed that at sea, you hope to avoid regions with
gentle hillocks of 50 ft wave height. :-)

Brian W