How can this be?
 

Several months ago I had a temporary U/VHF base antenna.
It was a MFJ-1729 dual band mag mount stuck to a radial
plate on a 30' mast. I was feeding it with about 36' of RG58
that was made up of 3 sections, joined with SO-239 barrels.

I thought it was working great because I could hear a friend
on the input to a repeater some 50 miles away (he lives very
close to the repeater).

I improved the antenna with a mono band vertical, better
coax & more mast. My math says I gained 10' in altitude
and 2.7 db overall (factoring in new coax & better antenna).

All of a sudden my new, better & higher antenna could NOT
hear my friend! WTF? I figured it was a band opening and
forgot about it.....

Last week the ice storms caused a branch to fall, ruining my
antenna. Due to $$$ shortage (replacing my HF rig), I opted
to go back to the temporary setup. The tip of the antenna is
10' lower and overall gain is 2.7db less.

I can hear my friend again!

I don't get it - the premium antenna worked better except for
hearing my friend, where it worked worse. Anyone care to offer
an explanation?

Ken KG0WX

Ken Bessler wrote:
I don't get it - the premium antenna worked better except for
hearing my friend, where it worked worse. Anyone care to offer
an explanation?

Perhaps there was constructive multi-path interference
on your friend's signal at the location of the lower
antenna but not at the location of the higher antenna -
or vice versa. Or your friend was not located in the
plane of maximum gain of the higher gain antenna. That
2.7 dB of extra gain at one take-off-angle comes at the
expense of reduced gain at a other vertical angles.
--
73, Cecil http://www.qsl.net/w5dxp


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"Ken Bessler" wrote in message
news:nPRFd.18091$ph.11198@okepread01...
Several months ago I had a temporary U/VHF base antenna.
It was a MFJ-1729 dual band mag mount stuck to a radial
plate on a 30' mast. I was feeding it with about 36' of RG58
that was made up of 3 sections, joined with SO-239 barrels.

I thought it was working great because I could hear a friend
on the input to a repeater some 50 miles away (he lives very
close to the repeater).

I improved the antenna with a mono band vertical, better
coax & more mast. My math says I gained 10' in altitude
and 2.7 db overall (factoring in new coax & better antenna).

All of a sudden my new, better & higher antenna could NOT
hear my friend! WTF? I figured it was a band opening and
forgot about it.....

Last week the ice storms caused a branch to fall, ruining my
antenna. Due to $$$ shortage (replacing my HF rig), I opted
to go back to the temporary setup. The tip of the antenna is
10' lower and overall gain is 2.7db less.

I can hear my friend again!

I don't get it - the premium antenna worked better except for
hearing my friend, where it worked worse. Anyone care to offer
an explanation?

Ken KG0WX

Several possiable casuses. Your friend started using lower power for a
while. The patern of the new antenna had a null in your friend's direction.

Hi Ken

Location, Location, Location and of course Take-Off Angle!

I had to lower a 55 foot antenna down to around 45 feet to hit our
local repeater only 3 miles away. At 55 feet it was blocked by a
microwave antenna, at 40 feet it is blocked by several steel framed
buildings. At 45 feet, there was a hole I could squeek through so to
speak.

Now, if I switched from a 5/8 wave to a 1/2 wave vertical, I had to go
all the way down to around 30+ feet to find this hole again.

TTUL
Gary

Nah-- Think is is referred to as " Fresnel Zones", and other
nomenclature- It has to do with multi-path - phase relationships
of the recieved signal- similar to the drive down a country road
at midnight, listening to an A.M. broadcast station (DX) fadeing
in-out - if you measure the distance between the 2 fades, it
equals 1/2 wavelength ! this effect works also in the verticle plane!
and, at vhf- the effect is quite noticeable in the fades (every 40
inchs at 2 meters, and about 18 inchs on 450) has to do with the
multi path signals arriveing in phase, or out of phase (cancelling
each other)! as info-- Jim - NN7K

Cecil Moore wrote:
Ken Bessler wrote:

I don't get it - the premium antenna worked better except for
hearing my friend, where it worked worse. Anyone care to offer
an explanation?


Perhaps there was constructive multi-path interference
on your friend's signal at the location of the lower
antenna but not at the location of the higher antenna -
or vice versa. Or your friend was not located in the
plane of maximum gain of the higher gain antenna. That
2.7 dB of extra gain at one take-off-angle comes at the
expense of reduced gain at a other vertical angles.

great explanation! I actually learned something. BTW, my answer was gonna be
,," majic"..


"Jim - NN7K" wrote in message
m...
Nah-- Think is is referred to as " Fresnel Zones", and other
nomenclature- It has to do with multi-path - phase relationships
of the recieved signal- similar to the drive down a country road
at midnight, listening to an A.M. broadcast station (DX) fadeing
in-out - if you measure the distance between the 2 fades, it
equals 1/2 wavelength ! this effect works also in the verticle plane!
and, at vhf- the effect is quite noticeable in the fades (every 40
inchs at 2 meters, and about 18 inchs on 450) has to do with the
multi path signals arriveing in phase, or out of phase (cancelling
each other)! as info-- Jim - NN7K

Jim - NN7K wrote:
Nah--

Nah?

this effect works also in the verticle plane!
and, at vhf- the effect is quite noticeable in the fades (every 40
inchs at 2 meters, and about 18 inchs on 450) has to do with the
multi path signals arriveing in phase, or out of phase (cancelling
each other)! as info-- Jim - NN7K

What you have described is constructive and destructive
interference - the same effect I described.

Cecil Moore wrote:
Perhaps there was constructive multi-path interference
on your friend's signal at the location of the lower
antenna but not at the location of the higher antenna -
or vice versa.
--
73, Cecil http://www.qsl.net/w5dxp


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