Topband: Receiving & transmitting antennas atop building

[Eduardo Araujo]

Hello Topbanders, Sorry and warning long append !!
asking for technical help.

This is my first append here, I promise next will be
very short. I am planning to be operative on 160 but
my environmental conditions are far from ideal, so,
the challenge is to try to get the most I can from it,
so please, I need your expertise help. By the way, I
did my home work, reading all posts since 1996 until
Nov 2004 trying to find experiences on high noisy
environment on big urban areas and with receiving
antennas like loops, pennants, etc, atop high
buildings.
 
First I will describe the environment and then let’s
split it into RX and TX antennas questions.
 
My antennas are atop a 14 floor building in a
neighborhood of Buenos Aires City. Main roof is 33 x
80 feet with another smaller one 15 x 30 x 12 high
atop, for the elevators machines and water tank.   
Atop the small roof, I have my 36 feet tower which is
currently supporting my 80 mts short rotary dipole (50
feet total length) with linear and coil loading, a 4
element yagy, and a full size inverted V for 40 mts.
Both roofs are covered with aluminum plate for
protection from the elements. 

Noise environment: 
I receive strong signals from broadcast that varies in
distance and power. As a reference, the worst
broadcast at 1030, produce +10dbm on my 80 mts dipole
terminated with 50 ohms, and +4dbm on my 40 mts
inverted V hanging from the tower at 30 feet level.
Another one put +6dbm and 4 others +1dbm; the ones at
1-4 miles -2dbm. Both antennas are feed with RG-213
via current baluns one at the antenna feed point and
the other at my shack. All coax are being replaced
this weekend by a double shielded for the 80 mts
dipole and RG8 Foam (with aluminum and braid) for the
all others antennas.

I am using a little modified ARRL HPF in the receiving
line of my ICOM 751A to reduce Am broadcast QRM. I
have a noise of S5 (SSB) on 80 during the night.
(PRE=OFF S9=50 uV measured) and S1-4 on 40 SSB with
Pre ON.

RX antenna.
I've been experimenting with coax loops on 80, looking
for outperforming my rotary dipole on receive, and
from there, scale it on 160.
I made a loop with 2.5 feet per side, the inner
conductor tuned by a capacitor, and the braid split. I
always try at about 3 feet from the main roof. 
I tried different feeds systems, a direct connection
and also a small loop inside the loop. 

I also tried the coax braid alone with a small
internal loop 1/5 of diameter and finally via a split
toroid passing the primary as 1 single turn and 6
turns as a secondary.

I tried it with and without a pre-amplifier, I tried
the amplifier at the antenna feed-point and also at
the shack.

Best results are with the amplifier (2N5109 24mA) at
the antenna feed-point thru a HPF and BPF for 80, but
under no circumstances the loop approached the dipole,
especially at long distance. Best case is 1-2 S units
poorer S/N ratio, other cases are worse. In no case it
has equal or better S/N with any distant station.

I tried many times with ANC-4 in all the ways I could
imagine (it could be I didn't imagine the one that
works) but I never was able to improve S/N ratio.
 
Pete. W2PM, provided me a description of a small
pennant (diamond configuration) suggested by K6SE and
also, in contact with Earl, provided me some tips, but
the result is it is still below the dipole (S/N). (I
wonder if it is happening because all this antennas
favor vertical polarization
 As a comment, every time
I try a vertical antenna, it has more noise than it
horizontal counterpart. 

At this time I feel a little frustrated but I will
keep on experimenting. 
Dear fellows:
- is anyone able to suggest what could be use for
receiving atop of a building in a high noise area to
improve S/N?
- Is it worth to try a normal size pennant (the limit
of my building roof)?
- Could any of you, simulate the behavior of these
receiving antennas atop a building trying to find a
key?
- I red Tom modified MFJ 1026 and said it is better
than ANC-4, Tom and other, is it worth to give it a
try?

TX Antennas
For transmitting I have a half wave wire hanging form
the building to the street (Pacific - South Pacific)
and I am planning to feed it at the end with a
parallel tuning circuit against 3 possible options
-	2 short horizontal radials parallel to 2 feet from
the roof.
-	1 quarter wave radial hanging 1 foot parallel from
the building to floor level. 
-	A connection to the building water plumbing at the
roof
Any suggestion?

The second transmitting antenna would be a quarter
wave hanging from the building to the street level
(Europe and USA), feed also at the end against short
radials and another type of feed tuning.
Any Suggestion?

Many thanks in advance for your patient and my
unshrinking English.

Best regards... Edy, LU2DKT



	
		
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