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Author Topic: 2 Meter simplex radio that's intermod-proof  (Read 37255 times)
KA2FIR
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Posts: 33




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« on: August 30, 2013, 09:49:22 PM »

Hi,

I live under a 2m repeater that had it's antenna upgraded and output increased which seems to be the cause of intermod on my TM-V71A. What surplus radios are out there that I can crystal or program for two meter simplex frequencies that will be less prone to front end overload from this repeater? I have an old TR-9130 that I may put back in service if it's receiver does not fail under these conditions.

Tnx,

Mike KA2FIR
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AF6WL
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Posts: 142




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« Reply #1 on: August 30, 2013, 10:08:04 PM »

Perhaps a short beam or pair of dipoles phased as a cardioid or figure of eight would give you sufficient null
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WB5ITT
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« Reply #2 on: August 30, 2013, 11:47:46 PM »

You say you live under it.....please clarify...is it on your home tower?? Or on a tower just down the street or Huh

Most amateur gear has such a wideband front end, they will intermod easily...if you want a simplex only radio, you should look at certain commercial rigs..
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AA4PB
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« Reply #3 on: August 31, 2013, 05:15:52 AM »

The issue is that the strong signal from the repeater and your simplex frequency are in the same band. Even with most commercial grade receivers the front end is going to cover most of the 2M band. You could add an external filter tuned to the frequency of repeater output but it will need to be very sharp (as in a tuned cavity) in order prevent it from also attenuating the simplex frequency.

There are some small filters available but they are intended to reject out of band signals whereas you need to reject an in band signal.

The one benefit of a commercial receiver that might help (depending on the strength of the repeater output signal) is that the front end could have more dynamic range therefore be able to better tolerate a strong in-band signal. It all depends on what signal level you have and how far away it is from your simplex frequency.
« Last Edit: August 31, 2013, 05:57:26 AM by AA4PB » Logged
KA2FIR
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Posts: 33




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« Reply #4 on: August 31, 2013, 11:20:28 AM »

You say you live under it.....please clarify...is it on your home tower?? Or on a tower just down the street or Huh

Most amateur gear has such a wideband front end, they will intermod easily...if you want a simplex only radio, you should look at certain commercial rigs..

I'm half way up a hill while the repeater is ontop of the hill. I would say I'm about 1/4 mile from the repeater so I'm down and to the right of it if facing the hill/mountain.

Mike KA2FIR
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K5LXP
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« Reply #5 on: August 31, 2013, 05:02:15 PM »

What exactly are the symptoms of this overload?

Mark K5LXP
Albuquerque, NM
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KB4QAA
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« Reply #6 on: August 31, 2013, 07:28:21 PM »

http://www.morganmfg.us/radio-products/individual-bandpass-filters/

Easy answer:  Get a bandpass filter.

Morgan Mfg sells the same filters and devices they used to make for ICE, before the owner died.

http://www.dci.ca/?Section=Products&SubSection=Amateur

http://www.parelectronics.com/amateur.php

Additionally 2m bandpass filter projects have been featured in QST and ARRL pubs.
« Last Edit: August 31, 2013, 07:37:09 PM by KB4QAA » Logged
W4OP
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« Reply #7 on: August 31, 2013, 07:45:14 PM »

You're not going to find a bandpass filter that will reduce a 2M repeater and still allow 2M simplex.
You might be able to configure a pair of full sized cavities in a  notch/pass configuration- but that is going to limit your simplex coverage in addition to seriously deplete ones wallet.
You might look at the early Radio Shack rigs.
Dale W4OP
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VA2PBJ
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« Reply #8 on: August 31, 2013, 10:37:01 PM »

When I lived in Halifax, I discovered that this place was intermod city. I found by securing a *really* good ground and using a narrower band antenna (5/8 gnd plane worked for me), my radios behaved much better.

Note that my source of intermod was likely all the commercial vhf traffic in the city, as it is a provincial capital and major naval and air bases within the cities. This spawns all the commercial support industries with their vhf usage.


If it still gets too bad, you can always put a tuned cavity notch filter set to the repeater transmitter.
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7 3 Peter VA2PBJ
WB2RXF
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« Reply #9 on: September 01, 2013, 12:58:05 PM »

You're not going to find a bandpass filter that will reduce a 2M repeater and still allow 2M simplex.
You might be able to configure a pair of full sized cavities in a  notch/pass configuration- but that is going to limit your simplex coverage in addition to seriously deplete ones wallet.
You might look at the early Radio Shack rigs.
Dale W4OP
Wrong... I've used a bandpass filter for my Kenwood TR-751A for many years, on 2 meters, while working long haul on ssb working stations 200miles away / satellites many times and could also work repeaters easily 100 miles away, when vertically polorized, and that was with a single Cushcraft 13B2, living across from lower manhattan, in north brooklyn, is no easy task, in my case it has blocked 95% of noise, while having a ball, it must have.  This one http://www.hamradio.com/detail.cfm?pid=H0-003431 Although I don't recall it being so expensive, its been more than 10 years Grin
« Last Edit: September 01, 2013, 01:23:45 PM by WB2RXF » Logged
AA4PB
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« Reply #10 on: September 01, 2013, 02:10:04 PM »

According to the specs the DCI-146-4H band-pass filter has a bandwidth of 4MHz centered on 146MHz. That means it is designed to pass everything from 144MHz to 148MHz (the whole 2M band). It'll stop signals outside of the 2M band but it won't allow you to work a simplex frequency while blocking a repeater output because they both fall within the filter's bandwidth.

A full sized cavity is probably the only way that you are going to block the repeater output while still allowing simplex frequencies to pass through.

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KB2VUQ
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« Reply #11 on: September 01, 2013, 03:56:06 PM »

I know it is a dumb question, but...have you tried using the A.I.P. on the V.H.F.
side of the Kenwood TM-V71a and if so, how well did that work?
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W4OP
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« Reply #12 on: September 01, 2013, 04:06:14 PM »

WB2RXF Wrote:
Wrong... I've used a bandpass filter for my Kenwood TR-751A for many years, on 2 meters, while working long haul on ssb working stations 200miles away / satellites many times and could also work repeaters easily 100 miles away, when vertically polorized, and that was with a single Cushcraft 13B2, living across from lower manhattan, in north brooklyn, is no easy task, in my case it has blocked 95% of noise, while having a ball, it must have.  This one http://www.hamradio.com/detail.cfm?pid=H0-003431 Although I don't recall it being so expensive, its been more than 10 years Grin
_____________________________
What was your source of intermod? In this case, it's a repeater, that at best may be 2.5MHz away. Please give me a link to a bandpass filter that has more than a couple dB rejection that close in.

Dale W4OP
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KA2FIR
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Posts: 33




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« Reply #13 on: September 03, 2013, 04:33:27 PM »

I know it is a dumb question, but...have you tried using the A.I.P. on the V.H.F.
side of the Kenwood TM-V71a and if so, how well did that work?

Not a dumb question. I have tried that feature and it helps but also removes weak signals that I still want to hear. My TR-9130 seems to be just as sensitive as the TM-V71A and seems to be less affected by the repeater but still causes white noise to appears on signals.

Mike KA2FIR
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KA2FIR
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« Reply #14 on: September 03, 2013, 05:38:45 PM »

The repeater is on 146.625 and I'm on 146.52. Looks like the DCI-146-4H will pass the whole 2m band so I don't think it's a solution.

Mike KA2FIR
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