Pager interferance again

[VK4BXI]

Ok my 2 metre all mode is getting nailed by pager signals at ~ 148Mhz.The signal is very strong ! I already have a band pass filter in and that's helping but I need to go to notch filters as well. HELP!! Is there anyone out there who has got a design for doing this or who has done it before. I have some lengths of heliax that could be used for 1/4 wave tuned lengths. I have a MiniVNA pro so should be able to tune a notch filter. According to rough measurements I'm going to need in the order of 40db total (the band pass is doing ~20db already but its a 144 to 146MHZ band pass filter so if I tune it for 144 to 148 MHZ like I need it to be its not going to help much.) http://www.vklogger.com/forum/posting.p ... =post&f=61# My QTH is just behind Dan Murphy's on Sumner road so its high and clear in most directions (Dan Murphy is up slope and north to me) I'd like to know where the pager site is to see if a physical screen will help. The rig is a flex 5000 with the V/U module the receiver is "hot" (their words) and the problem is better if the preamp is switched out. So I think that I may be looking at three stages of notch (~15db of notch per tuned 1/4 wave stub ?) Comments and advice rude or otherwise are welcome.

Regards

bob

[VK4TI]

Ok my 2 metre all mode is getting nailed by pager signals at ~ 148Mhz.The signal is very strong ! I already have a band pass filter in and that's helping but I need to go to notch filters as well. HELP!! Is there anyone out there who has got a design for doing this or who has done it before. I have some lengths of heliax that could be used for 1/4 wave tuned lengths. I have a MiniVNA pro so should be able to tune a notch filter. According to rough measurements I'm going to need in the order of 40db total (the band pass is doing ~20db already but its a 144 to 146MHZ band pass filter so if I tune it for 144 to 148 MHZ like I need it to be its not going to help much.) http://www.vklogger.com/forum/posting.p ... =post&f=61# My QTH is just behind Dan Murphy's on Sumner road so its high and clear in most directions (Dan Murphy is up slope and north to me) I'd like to know where the pager site is to see if a physical screen will help. The rig is a flex 5000 with the V/U module the receiver is "hot" (their words) and the problem is better if the preamp is switched out. So I think that I may be looking at three stages of notch (~15db of notch per tuned 1/4 wave stub ?) Comments and advice rude or otherwise are welcome.

Regards

bob

I seem to remember there is a large comms tower quite close to you Bob so best to determine a couple of things , first have you tried another rx , I expect there is some imd involved and have you taken a look at the pager signals as often they are driven wide and nasty so it may well be the tx and not you at fault in which case you would have a basis for complaint , there are plenty of high q stub designs available from qtz and other sites on a google , good luck and keep us informed please but in the meantime talk to Doug VK4OE as he has intimate knowledge of the matter .

B

[VK3ZAZ]

sections of 1/4 wave rg58 and bnc tees trim the cable you will pull the pager into noise'
USe two if necessary
separated by 1.4 section
or get an old cavity with bnc tee suck it out that way but it will screw top end of 147 mhz.
we have filters on site that receive a long distance transmission on fm receiver despite five 100kw services on same tower

[VK4BXI]

Yes I have tried other RX's....I have a FT897 in place which is about the same if not worse. I am doing the detective work with a fun dongle + SDR. To receive without distortion I have the pre amp and the if amps both turned off and still I'm at -30db peaks. And yes the signals are wide and nasty with two of them going almost but not quite in tandem. I will take some screen shots and post them if I can for the groups comment.
I'm not too sure about using RG58 as stubs on the aerial line...the Flex 5000 runs 50 watt+. I'm quite happy to use RG58 for stub separation but where does 1.4 factor come from ? is that wavelength ? I have read about using a1/4 wave length to separate stubs so that they don't interfere with each other...did you mean 1/4 and missed the divide key ?

Regards Bob

[VK7DB]

http://www.vk5zd.com/PagerFilter/Filter.aspx

[VK6ZFG]

Hi Bob

What you need is a notch filter with a very high Q. Otherwise you will suffer from loss in the top end of the 2M band. Coax based filter are pushing up hill to meet this requirement. The insulation and small diameter all contribute to the low Q. Temperature stability is also another factor which tends to be overlooked with these.

What you need is a cavity filter, bigger diameter the better. Use this as a notch filter. For even better results look at at 2M repeater diplexers made with cavities and inductance/capacity across the cavity ports. You need say one cavity filter (or more) as used on on the transmitter leg as this provides maximum attenuation on the high frequency side but tends to act as a low pass filter on the low frequency side.

Have a look at the frequency separation between the offending pager signal and the highest frequency what you want to use. This is what the notch filter needs to cater for at a loss that is acceptable to you.

Best of luck

[VK7DB]

http://www.repeater-builder.com/antenna ... plexer.pdf

[VK2JDH]

A ACMA lookup for around your postcode 4070 - 4079 reveals

http://web.acma.gov.au/pls/radcom/assig ... nge_lookup

Results 1 - 5 of 5 assignments.

ID Frequency Emission Designator Client Licence Number

135306-27023 148.3375 MHz 16K0F2D Tricare Ltd (411247) 425985
1410940-1408618 148.3625 MHz 16K0F2D Dept of Health Qld (20004872) 1421931
1422311-1417665 148.9875 MHz 16K0F2D Dept of Health Qld (20004872) 1426684
8234611-8252146 148.5625 MHz 16K0F2D Vodafone Hutchison Australia Pty Limited (20012076) 444701
8395141-8450636 148.6375 MHz 16K0F2D Vodafone Hutchison Australia Pty Limited (20012076) 444969

[VK4BXI]

Oh that's lovely.
It looks like the sites that are giving me grief are ~5km away but in clear view across the valley. Vodacom would seem to be my problem but now to check the exact frequency's. Back in the "good old, bad old" South Africa this kind of info is restricted !! and it is illegal to monitor other frequency's for interference !!##!!

Regards and thanks Bob

[VK4TI]

Oh that's lovely.
It looks like the sites that are giving me grief are ~5km away but in clear view across the valley. Vodacom would seem to be my problem but now to check the exact frequency's. Back in the "good old, bad old" South Africa this kind of info is restricted !! and it is illegal to monitor other frequency's for interference !!##!!

Regards and thanks Bob

You will be shocked by what information is commonly available , sure to be useful and you will sort it shortly I expect once the information is absorbed

[VK2JDH]

Oh that's lovely.
It looks like the sites that are giving me grief are ~5km away but in clear view across the valley. Vodacom would seem to be my problem but now to check the exact frequency's. Back in the "good old, bad old" South Africa this kind of info is restricted !! and it is illegal to monitor other frequency's for interference !!##!!

Regards and thanks Bob

If you look at the detail on the license for those 2 that are running 250 - 500 W ERP and are meant to have a cavity filter in the TX output

One has: 500W TX The level of all discreet spurious components, measured at the output of the transmitter, must not exceed -30dBm. The level of power in the adjacent channel must not exceed -22dBm.

The other: 250W An efficient cavity filter must be fitted between the transmitter and the antenna.

[VK6ZFG]

Some time back I recycled a Vodafone paging transmitter to be a narrow cast high band transmitter for Radio Lolipop. The paging transmitter had a crystal filter on the operating frequency in the exciter chain close to the PA so as to minimise adjacent frequency outputs.

Amateur radio equipment chases weak signal performance. This is achieved at the expense of strong signal performance.

Commercial communications equipment is required to be able to cope with strong signals so as to minimise reception interference problems. This is achieved by trading off weak performance to be able to cope with strong signals.

Unfortunately you can't have it both ways.

[VK4BXI]

-30dbm is still a reasonable signal. It's funny, I wonder if those people who run such sites, monitor these sorts of chat's on Ham web sites ? The signals are still there but somehow don't seem to causing so much of a problem. I didn't have an absolute signal level on them so I can't do a before and after but ? Maybe I'm just paranoid. I'm still going to go build myself a couple of notch filters.
It seems that 1/2inch heliax won't cut it and I need several 1/2 metre long lengths of the 2 inch/50mm stuff. I wonder if there is any out there being chopped up for scrap ?
Thanks to all who have replied and helped.

Regards Bob

[VK4TI]

-30dbm is still a reasonable signal. It's funny, I wonder if those people who run such sites, monitor these sorts of chat's on Ham web sites ? The signals are still there but somehow don't seem to causing so much of a problem. I didn't have an absolute signal level on them so I can't do a before and after but ? Maybe I'm just paranoid. I'm still going to go build myself a couple of notch filters.
It seems that 1/2inch heliax won't cut it and I need several 1/2 metre long lengths of the 2 inch/50mm stuff. I wonder if there is any out there being chopped up for scrap ?
Thanks to all who have replied and helped.

Regards Bob

Hard to come by but if you need some 450 I have a little used your welcome to as a just in case case
Its possible to build something suitable from plumbing copper at some cost of course

[VK4TIM]

If you're interested, I have two cut off tails of LDF5-50 that I can give you to experiment with, these are probably approximately 1.2m in length, removed from a decommissioned comemrcial site. You never know when this stuff might come in handy, so I hung onto it.

You might be able to make two bandpass or notch filters with these.

Have you been able to determine which pager TX that the interference problem is coming from? are you affected by some spurious emission from the pager TX? (Most of the pagers are probably 20yr old Motorola gear, and drying electrolytics in the PA bias supply can oscillate and AM the PA driver..Seen it more than once), does the problem vary with temperature or humidity? Are you only receiving the interference when two or more transmitters come up at once?

[VK6ZFG]

Hi Bob

just did some tests on a length of 4/50 coax I had lying around (316 mm long to end of N type connector).

Test Setup

Unfortunately the results don't look all that promising.

Results

Loss at :
100.0 MHz -0.7dB
183.7 MHz -28dB
184.7 MHz -32dB
185.7 MHz -40dB

Sweep 0 to 200 MHz with sweep normalised over the sweep range.

RG58 had an inferior null to the 4/50. 5/50 will have a deeper, sharper null.

The stub will have an impact on the VSWR.

Someone once said life was not meant to be easy (to which I add "Then made sure it was not.")

[VK4BXI]

Guys I'm still getting there......I've got some 1/2" heliax....same as 4_50 I think. and as noted a couple of tuned lengths don't really cut it (Hi Hi). A couple of the articles that have been shared here look very promising and either I spend the time making a "proper" cavity or a quick and dirty solution with something like 5_50 or better The article used 50mm diameter (AVA 7-50) if I'm reading my Andrews cat correctly !!. I do know which pagers have been offending me, given the excellent information available on the net, its definitely two Vodacom transmitters on the same site close to me. What I'm going to do now is to make a good notch filter to take them out. If there are still issues then the next thing to do is to look at what is left for intermod etc and decide if there is merit in going further.

Regards Bob

[VK3ZAZ]

IGOR said
The stub will have an impact on the VSWR.
ERR yes in a real world filters have insertion loss and attenuation even best made ones.

I always tell the story of a TV Site in SW VIC
CFA EMS moved in with a 250 w pager on site on 148.025
We had transposer on site receiving 137-144 MHZ CH5A

After they wiped out the whole town when pager keyed up we called ACMA

They came to site, observed the great big carrier from the pager capturing our TV translator and said..

You have bloody great carrier in your bandpass capturing your RBR off air rx.

And we said what do we do about it..

They said stuffed if we know and by the way its going 500 watts soon.,

We bought $500 filter to no avail

So we spent $50K and put in a bloody great satellite dish...

So if its really important to block the pagers at 148 MHz running 500 watts and work weak signals on 2M

MOVE

sorry could not resist.

[VK4TI]

Guys I'm still getting there......I've got some 1/2" heliax....same as 4_50 I think. and as noted a couple of tuned lengths don't really cut it (Hi Hi). A couple of the articles that have been shared here look very promising and either I spend the time making a "proper" cavity or a quick and dirty solution with something like 5_50 or better The article used 50mm diameter (AVA 7-50) if I'm reading my Andrews cat correctly !!. I do know which pagers have been offending me, given the excellent information available on the net, its definitely two Vodacom transmitters on the same site close to me. What I'm going to do now is to make a good notch filter to take them out. If there are still issues then the next thing to do is to look at what is left for intermod etc and decide if there is merit in going further.

Regards Bob

With luck you can check the spread of the pager and then complain as it might well be out of spec ? .
or someone will check it for you ? , spec an would help

[VK7DB]

Hi Bob

just did some tests on a length of 4/50 coax I had lying around (316 mm long to end of N type connector).

Picture 029r.jpg

Unfortunately the results don't look all that promising.

Picture 030r.jpg

Loss at :
100.0 MHz -0.7dB
183.7 MHz -28dB
184.7 MHz -32dB
185.7 MHz -40dB

Sweep 0 to 200 MHz with sweep normalised over the sweep range.

RG58 had an inferior null to the 4/50. 5/50 will have a deeper, sharper null.

The stub will have an impact on the VSWR.

Someone once said life was not meant to be easy (to which I add "Then made sure it was not.")

I think this would work better if it was done more like the 6m duplexer designs..

http://naomi.mountains.net.au/hamradio/ ... plexer.pdf