since 23rd Jan, 2000
Ham radio spoken here
(last updated 5th Sep, 2000)
Mail to dominicb@lucent.com. SPAM.ME.NOT <-- remember to remove the .SPAM.ME.NOT
The information supplied here is simply for interest or to aid another interference sufferer track down their problems. The intent is simply to give an example of how we went about tracking down the interference & doing some pre-work to enable the authorities to help us easily.
Our main 2 metre voice repeater (146.875MHz -600kHz) was subject to some sporadic interference which was annoying to most users.
A = Voice/music frequency = 151.450 MHz
B = pager frequency = 148.8625 MHz
C = VK2RMB Rx frequency = 146.275 MHz
2B - A = C (if this equation holds, then there could be a problem)
(2 X 148.8625) - 151.450 = 146.275 (it did!)
or, another way of looking at it...
D = A - B = 151.450 - 148.8625 = 2.5875 MHz ( difference frequency )
B - D = 148.8625 - 2.5875 = 146.275 MHz
Armed with this & other evidence from other club members who listened & compiled notes, we filled in an Australian Communications Authority (ACA) complaint form RF66 and attached a letter of explanation (with our investigations). I telephoned them to determine the most appropriate person before faxing all this in.
Opening up the club rooms when the ACA investigator arrived, we plugged in the aerial (via the 3 Rx cavity filters) to the spectrum analyzer & watched the screen until we saw the interfering signal. This took a fair while since were were at the club rooms at 7:30am! The level of the interfering signal was not strong but was over -110dbm. This level was enough to come through with weak (ham) input signals, but not enough to key up the repeater by itself at that time.
Whilst there we did a sweep of the cavity filters that we have installed, just to check that they were working adequately. We confirmed that they were working quite well for home-made filters, although a little down on performance compared to what would be expected from professional cavity filters. "Perfectly adequate". We have drawn the sweep output of the diplexer cavities (6) & the bandpass (1) cavity filters & will be shown here!
(After a week) This particular interference problem seems to have gone away. I have just confirmed this with the ACA & will pass on our thanks to the pager owner for their quick co-operation.
We were told by the ACA investigator that we had probably saved them weeks
of work in tracking down the interference by the pre-work we had done. The
problem was not at our site & so we had no payments to make to the ACA for
handling this problem & we found the ACA more than helpful & polite.
Date: 21 December 1999.
I am Dominic Bragge VK2JNA
President,
Manly-Warringah Radio Society
Note: text in brackets has been added for clarification in the quotations.
Interference description
------------------------
With reference to the 146.275MHz receiver attached to the 146.875MHz
transmitter (Amateur repeater VK2RMB at Terrey Hills)
PROBLEM
We are receiving interference on the receive frequency which seems to be
not of `amateur' origin. This keys-up the repeater to transmit the
received signal. The signal is a combination of voice & music.
HOW OFTEN
It is intermittent & seems to happen more during business hours on Mon-Fri
but not limited to this time frame. This problem has been going on for a
couple of months.
OUR GUESS
We currently believe that there is a third-order intermodulation problem
with the frequencies 151.450MHz & 148.8625MHz appearing on our repeater
input of 146.275MHz. We don't believe that the mixing is occuring at our
site.
PERTINENT INFORMATION (possibly)
1. One of our club members has identified a signal that has the same audio
that is being received by our repeater. It is on 151.450MHz.
2. A second member looked up on the ACA database (internet), based on the
frequency 151.450, to find that it might be a transmitter located near
Bankstown that is of interest, which details can be viewed at...
http://www.aca.gov.au/cgi-bin/assignment_detail?access_id=1205572&dev
ice_id=1203798
...of which the pertinent information is...
Client Number: 1209701
Licence Number: 1204245
Access_id: 1205572
Operating Mode: Transmit
Assigned Frequency 151.4500000 MHz
Date Approved: 14 October 1999
Coverage: Local
Device_id: 1203798
Emission Designator: 16K0F3E
EIRP: 0
Transmitter Power: 50.00 pY
Antenna_Id: 70038
Antenna Height (AGL): 20
Antenna Polarity: V
Antenna Azimuth: ND
Site_Id: 5184
He went out there in his car with his amateur receivers and found...
"With the fact I could remove my antenna and then only hear the
(151.450MHz) signal (on 146.275MHz) within a 1 km radius says it is the
151.450 xmitter itself, OR an intermod with another service that is
also within the 1 km radius..."
3. The same second member did some more looking into the problem...
" This time I took two handhelds to confirm what I heard on my mobile
transceiver. On the input 146.275 (I) can hear the source signal from
151.450 while driving around Yagoona (and) I noticed it is only heard
50% of the time but a lot more than (it) is heard via the repeater.
At (what I think is) the site of 151.450, I removed all antennas & can
hear the signmal on 146.275 quite clear, still only 50% of the time,
while monitoring 146.275 searched on another receiver and found a pager
on 148.8625 which matches up against the periods where the music is
heard on 146.275.
Listening to 151.450 it does have an intermittent crackling, but it did
not seem to co-incide with the periods when it is heard on 146.275.
I found a few other pagers which were also very strong even without an
antenna but the 148.8625 one was different in that it had a clean
carrier tail sometimes but the signal strength of the tail would vary
from strong to weak and maybe the (148.8625) transmitter was not
shutting off at the end of the data burst.
(I) drove back to Homebush with the antennas now on and could still
match 148.8625 activating & producing the signal from 151.450 onto
146.275 ".
4. A third member has taken some notes (whilst monitoring on the air)
and they are as follows...
" Notes re: Interference to VK2MB repeater.
29-10-99
First appearance of the interference at 5.50 am.EST Some hash with the
signal from 151.450 Music from 151.450 co-incidental with direct
signal.
2-11-99
Throughout the day, on MANY occasions, the same varying signal
strength. Listening to the signal from 151.450 directly, it also
varies.
At 5.50am.DST, there was just the crackle like a strong unmodulated
signal, until 5.57am, when the signal was modulated with music etc.
This was a direct signal, but not as yet breaking into 146.275.
At 6.06am. a few intermittent bursts did trigger the repeater.
Frequently, the rptr is triggered with only the hash audible. Not the
modulated signal, although that may only be distance and the varying
signal.
6.45 am. Hash and signal from 151.450 both heard via repeater. Then
frequently, very much varied in time intervals. From 30 secs to an
hour.
4-11-99
It is suggested that a note or recording etc should be made of the
strong signal on the 151.450 freq. This should be done before
5.50amDST. Before the modulation starts is a very strong carrier which
is VERY noisy with a broken static type sound. This alone does not
trigger the repeater ".
----
We request that you help us in solving this interference. I hope that
these notes are helpful & do not lead you off the track. Please be in
touch if we are likely to incur any expenses or if you simply wish to
discuss this problem at any time.
To Contact me:
(blah-deleted)
Yours sincerely
Dominic Bragge VK2JNA
acting as President of the Manly-Warringah Radio Society VK2MB.