A 2 Meter 5/8 Wave antenna is too long for a magnet mount.

Not if the magmount is one of those which is around 7" diameter. Also, with
the one I have, the coil at the base is something like 4 turns of 1" internal
diameter spring steel so, if the whip does hit something, there's a lot of
'twang' in the system. There's almost no way that the magnet can be
dislodged.

I stand corrected. Thanks.

http://www.hamcity.com/detail.aspx?ID=786

If you go to page http://www.kathrein.de/de/car/montag...ad/936127a.pdf
(the English description follows the German description) you will see the
144-MHz Kathrein antenna model K 51 17 2, which is a 5/8 wavelength
magnetic-mount antenna with a swivel. The whip is 1.38 m long (about 4.6 feet)
and the magnet diameter is 95mm (about 3.7 inches). The tuning coil is located
inside the mount..

That antenna is very common in Europe; they sell it since more than 30 years.
Please note that 57% of the German highways have no speed limit (see
http://en.wikipedia.org/wiki/Speed_limit) and it is not uncommon to see the big
Mercedes or BMW running at more than 120 mph).

Another 144-MHz 5/8 wavelength antenna I have has a magnet of comparable
diameter.

73

Tony I0JX

Ken,
If you consider how much movement that there is in a whip in a mobile
situation, the 20 deg will not make much difference to it, I bent mine a
fraction more, and it made little difference to the VSWR or the performance.

John VK2KC

"Chevy454" wrote in message
...
I want to put my 2M 5/8-wave mag-mount on the trunk lid, which slopes
down at 20 degrees. I am concerned that most of my signal is going
into the ground or outer space.

Can I bend the long whip so it stands vertically? Should I do it?
does the downward slope of the ground plane leave a problem
unaddressed?

Ken KC2JDY

On Jan 31, 5:01*am, Ian Jackson
wrote:
In message , Chevy454
writes

In message , dave
writes
But why the trunk lid? Surely the roof is better? On a vehicle, every
inch above the ground helps.

The car is being used for 2M fox hunting. *The Doppler antennas are on
the roof. *The MFJ-1728B is on the trunk lid.

Are you saying that the 6 dB gain of this antenna is not high enough
for a 20 degree tilt to make a difference?

Ken KC2JDY

Noted about the roof already being occupied.

Obviously, if the antenna is 'leaning backwards' at 20 degrees, the
radiation in the forward direction is going to point upwards more than
it would if ie was vertical and, of course, to the rear, the radiation
will point more downwards. However, even with the antenna dead vertical,
the shape of the body of the vehicle will distort the radiation pattern
considerably. I've had a quick Google, and this is one site which
provides some info:
http://www.k6mhe.com/files/mobile_vhf_ant.pdf
There will be lots of others.

This also might be of interest:
http://74.125.77.132/search?q=cache:...st.com.au/Ante
nnas/overview.html+antenna%2Bradiation%2B%225/8+wavelength%22&hl=en&ct=cl
nk&cd=25&gl=uk

As I said, you probably won't notice much difference between vertical
and 20 degrees. But the only way to find out will be to try it.
--
Ian

I think I remember from some place that tilting a vertical monopole
operating against a counterpoise increases the take off angle in both
the direction it is tilted and the direction opposite.

I welcome correction if I am in error.

Jimmie

On Fri, 30 Jan 2009 12:17:57 -0500, Chevy454 wrote:

I want to put my 2M 5/8-wave mag-mount on the trunk lid, which slopes
down at 20 degrees. I am concerned that most of my signal is going
into the ground or outer space.

Not that big of a deal. What is (1 - the cosine of 20 degrees)?


Can I bend the long whip so it stands vertically? Should I do it?
does the downward slope of the ground plane leave a problem
unaddressed?

Ken KC2JDY

It will probably upset the grip of the magnetic mount a bit. It will
probably give you a curved radiator. Not much effect, try it if you
like.

In message
,
JIMMIE writes
On Jan 31, 5:01*am, Ian Jackson
wrote:
In message , Chevy454
writes

In message , dave
writes
But why the trunk lid? Surely the roof is better? On a vehicle, every
inch above the ground helps.

The car is being used for 2M fox hunting. *The Doppler antennas are on
the roof. *The MFJ-1728B is on the trunk lid.

Are you saying that the 6 dB gain of this antenna is not high enough
for a 20 degree tilt to make a difference?

Ken KC2JDY

Noted about the roof already being occupied.

Obviously, if the antenna is 'leaning backwards' at 20 degrees, the
radiation in the forward direction is going to point upwards more than
it would if ie was vertical and, of course, to the rear, the radiation
will point more downwards. However, even with the antenna dead vertical,
the shape of the body of the vehicle will distort the radiation pattern
considerably. I've had a quick Google, and this is one site which
provides some info:
http://www.k6mhe.com/files/mobile_vhf_ant.pdf
There will be lots of others.

This also might be of interest:
http://74.125.77.132/search?q=cache:...st.com.au/Ante
nnas/overview.html+antenna%2Bradiation%2B%225/8+wavelength%22&hl=en&ct=cl
nk&cd=25&gl=uk

As I said, you probably won't notice much difference between vertical
and 20 degrees. But the only way to find out will be to try it.
--
Ian

I think I remember from some place that tilting a vertical monopole
operating against a counterpoise increases the take off angle in both
the direction it is tilted and the direction opposite.

I welcome correction if I am in error.

Jimmie

No. You are correct. At VHF/UHF, with a vertical antenna over ground,
the strongest radiation is always at an angle upwards (say 10 to 40
degrees, depending on the type of antenna). If you tilt the antenna
over, the angle of radiation is therefore lower in the direction of the
tilt, and higher in the opposite direction. On a vehicle, a whip with a
'sporty' rearwards rake will probably radiate somewhat better towards
the back. However, I wouldn't lose too much sleep about it.

As for the increased radiation in the direction of a counterpoise (or
radial), it is in that direction that the counterpoise provides a better
'mirror image' of the antenna. This augments the radiation in that
direction.
--
Ian

In message , JosephKK
writes
On Fri, 30 Jan 2009 12:17:57 -0500, Chevy454 wrote:

I want to put my 2M 5/8-wave mag-mount on the trunk lid, which slopes
down at 20 degrees. I am concerned that most of my signal is going
into the ground or outer space.

Not that big of a deal. What is (1 - the cosine of 20 degrees)?


Can I bend the long whip so it stands vertically? Should I do it?
does the downward slope of the ground plane leave a problem
unaddressed?

Ken KC2JDY

It will probably upset the grip of the magnetic mount a bit. It will
probably give you a curved radiator. Not much effect, try it if you
like.

Indeed. At speed, a nice, whippy stainless steel whip curves a scimitar.
Very pretty! But you'd have to be approaching Mach 1 to dislodge a 7"
limpet magmount.
--
Ian

"Ian Jackson" wrote in message
...
In message
,
JIMMIE writes
On Jan 31, 5:01 am, Ian Jackson
wrote:
In message , Chevy454
writes

In message , dave
writes
But why the trunk lid? Surely the roof is better? On a vehicle, every
inch above the ground helps.

The car is being used for 2M fox hunting. The Doppler antennas are on
the roof. The MFJ-1728B is on the trunk lid.

Are you saying that the 6 dB gain of this antenna is not high enough
for a 20 degree tilt to make a difference?

Ken KC2JDY

Noted about the roof already being occupied.

Obviously, if the antenna is 'leaning backwards' at 20 degrees, the
radiation in the forward direction is going to point upwards more than
it would if ie was vertical and, of course, to the rear, the radiation
will point more downwards. However, even with the antenna dead vertical,
the shape of the body of the vehicle will distort the radiation pattern
considerably. I've had a quick Google, and this is one site which
provides some info:
http://www.k6mhe.com/files/mobile_vhf_ant.pdf
There will be lots of others.

This also might be of interest:
http://74.125.77.132/search?q=cache:...st.com.au/Ante
nnas/overview.html+antenna%2Bradiation%2B%225/8+wavelength%22&hl=en&ct=cl
nk&cd=25&gl=uk

As I said, you probably won't notice much difference between vertical
and 20 degrees. But the only way to find out will be to try it.
--
Ian

I think I remember from some place that tilting a vertical monopole
operating against a counterpoise increases the take off angle in both
the direction it is tilted and the direction opposite.

I welcome correction if I am in error.

Jimmie

No. You are correct. At VHF/UHF, with a vertical antenna over ground, the
strongest radiation is always at an angle upwards (say 10 to 40 degrees,
depending on the type of antenna). If you tilt the antenna over, the angle
of radiation is therefore lower in the direction of the tilt, and higher
in the opposite direction. On a vehicle, a whip with a 'sporty' rearwards
rake will probably radiate somewhat better towards the back. However, I
wouldn't lose too much sleep about it.

As for the increased radiation in the direction of a counterpoise (or
radial), it is in that direction that the counterpoise provides a better
'mirror image' of the antenna. This augments the radiation in that
direction.
--
Ian

.... but, surely, there will be more counterpoise to the front of a whip
mounted on the boot/trunk lid so this should favour low-angle radiation
towards the front - shouldn't it? If true, the result of tilting the whip
back could be closer to onmi-directional at low elevations!

Chris

In message , christofire
writes

"Ian Jackson" wrote in message
...
In message
,
JIMMIE writes
On Jan 31, 5:01 am, Ian Jackson
wrote:
In message , Chevy454
writes

In message , dave
writes
But why the trunk lid? Surely the roof is better? On a vehicle, every
inch above the ground helps.

The car is being used for 2M fox hunting. The Doppler antennas are on
the roof. The MFJ-1728B is on the trunk lid.

Are you saying that the 6 dB gain of this antenna is not high enough
for a 20 degree tilt to make a difference?

Ken KC2JDY

Noted about the roof already being occupied.

Obviously, if the antenna is 'leaning backwards' at 20 degrees, the
radiation in the forward direction is going to point upwards more than
it would if ie was vertical and, of course, to the rear, the radiation
will point more downwards. However, even with the antenna dead vertical,
the shape of the body of the vehicle will distort the radiation pattern
considerably. I've had a quick Google, and this is one site which
provides some info:
http://www.k6mhe.com/files/mobile_vhf_ant.pdf
There will be lots of others.

This also might be of interest:
http://74.125.77.132/search?q=cache:...st.com.au/Ante
nnas/overview.html+antenna%2Bradiation%2B%225/8+wavelength%22&hl=en&ct=cl
nk&cd=25&gl=uk

As I said, you probably won't notice much difference between vertical
and 20 degrees. But the only way to find out will be to try it.
--
Ian

I think I remember from some place that tilting a vertical monopole
operating against a counterpoise increases the take off angle in both
the direction it is tilted and the direction opposite.

I welcome correction if I am in error.

Jimmie

No. You are correct. At VHF/UHF, with a vertical antenna over ground, the
strongest radiation is always at an angle upwards (say 10 to 40 degrees,
depending on the type of antenna). If you tilt the antenna over, the angle
of radiation is therefore lower in the direction of the tilt, and higher
in the opposite direction. On a vehicle, a whip with a 'sporty' rearwards
rake will probably radiate somewhat better towards the back. However, I
wouldn't lose too much sleep about it.

As for the increased radiation in the direction of a counterpoise (or
radial), it is in that direction that the counterpoise provides a better
'mirror image' of the antenna. This augments the radiation in that
direction.
--
Ian

... but, surely, there will be more counterpoise to the front of a whip
mounted on the boot/trunk lid so this should favour low-angle radiation
towards the front - shouldn't it? If true, the result of tilting the whip
back could be closer to onmi-directional at low elevations!

Quite possibly. Yet again, possibly not!

On my Mk3 Ford Escort hatchback, I had a 10m whip (1.5m base-loaded
DV27) mounted on the top lip (where the hinges are) in the centre. It
leaned back at about 20 degrees. I didn't notice that it was
particularly directional in any direction.

On 2m, I used to slap my 5/8 w/l on its magmount (gently, with its
rubber boot fitted) dead in the centre of the roof. When stationary, it
was (of course) vertical. But even though it would curve backwards at
speed, I wasn't aware of any obviously directionality.
--
Ian

I suggest that some of the people wondering about this download the free
EZNEC demo program from http://eznec.com, or any other modeling program.
This is just the sort of question these programs can answer. Judging by
a lot of the postings here, quite a few of you will be surprised by the
results and should find it educational.

Roy Lewallen, W7EL