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eHam Forums => VHF / UHF => Topic started by: KA2FIR on August 30, 2013, 09:49:22 PM



Title: 2 Meter simplex radio that's intermod-proof
Post by: KA2FIR 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


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: AF6WL 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


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: WB5ITT 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 ???

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..


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: AA4PB 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.


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: KA2FIR 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 ???

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


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: K5LXP on August 31, 2013, 05:02:15 PM
What exactly are the symptoms of this overload?

Mark K5LXP
Albuquerque, NM


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: KB4QAA 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.


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: W4OP 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


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: VA2PBJ 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.


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: WB2RXF 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 ;D


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: AA4PB 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.



Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: KB2VUQ 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?


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: W4OP 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


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: KA2FIR 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


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: KA2FIR 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


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: W4OP on September 03, 2013, 06:04:22 PM
As in my previous reply, there is no BP filter that will solve that issue.

AIP is a fancy word for attenuator- so your results are what one would expect on weak signals.

Dale W4OP


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: AA4PB on September 03, 2013, 06:50:42 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

That's only 105kHz, less that the normal 600kHz separation between a repeater input and output pair. It's going to be really tough to find a filter that will pass one and reject the other.


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: WB5ITT on September 03, 2013, 11:03:52 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.
Bandpass filters are wideband...they are NOT meant for a close frequency use....All they do is improve the out of amateur band rejection..but his problem is a HAM signal just up the road...a NOTCH on that freq would be better...depending on how close he is, may NOT be able to get rid of it all....of course a notch will affect adjacent channels.....I would say get the repeater moved!! Or move your station and operate it by remote control.....There is no easy fix for this one.

Chris
WB5ITT


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: AF6WL on September 04, 2013, 07:38:45 AM
...I would say get the repeater moved!! Or move your station and operate it by remote control.....There is no easy fix for this one.

The thing is neither your QTH or the repeater is likely to move and filtering is impractical.
That's why I suggested in the very first reply using an antenna null not reduce the signal from the repeater.

A simple and small directional antenna such as a moxon, which has a good Front to Back ratio, would be easy to build to test the principle.

http://www.moxonantennaproject.com/wb5cxc/wb5cxc.htm (http://www.moxonantennaproject.com/wb5cxc/wb5cxc.htm)


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: WB2WIK on September 04, 2013, 10:44:16 AM
I read the replies and agree "no" kind of bandpass filter will help when the frequencies are so closely spaced.

A very high-Q notch filter to notch the repeater's frequency specifically might help a little bit, but the Q required is so high I think the only way to achieve it might be an RF crystal filter (they do exist for VHF -- very expensive, and can't handle transmitter power, so would have to be "wired into" the receiver).

However, some receivers are much better at simply handling strong signals than others.

I don't recall how the TR-9130 did, but I had an old TR-7400A (1978 vintage!) that was dumb as a box of rocks: No memories, no scanning, no out-of-band coverage at all, and didn't even have CTCSS (that could be added separately).  But it was very, very immune to overload, about as good as more costly stuff I had from Motorola.  That is, it could simply handle a 1000uV+ signal just off frequency from a 1uV signal and the stronger one wouldn't affect reception of the weaker one.  "Most" 2m "ham" gear can't do that.  But it could.

Another thing I didn't see mentioned previously:

If you're down the hill and a bit away from the offending signal source (repeater), it might be possible to move your antenna slightly to null the repeater a bit, by out-of-phase cancellation.  Where there are hills, there are reflections, and they can be used to your advantage.  They're what cause the "phasing nulls" we've all seen operating VHF mobile...where you can roll the car into a location where even a strong signal just disappears, and then roll it another foot or two and it comes right back, strongly.  That's a phasing null, where the signal arrives by multiple paths and at some locations they'll arrive so perfectly out of phase, they'll just cancel and result in "no" signal.

Moving a fixed station antenna around to specifically null the signal from another fixed station antenna is possible, and I've done this quite often.


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: K1DA on September 05, 2013, 02:28:08 PM
THe Kenwood TK series of commercial grade radios have been used AS repeater receivers for a some time with good results.  The "wideband" versions hit Ebay and other sources when the commercial bands were "refarmed for a hundred bucks or less.  "Wideband"  still works for ham VHF FM.  Look at the KA!RCI website for some information or just google " Kenwood TK commercial"  Among other things the TKs have a TWENTY WATT audio output amp and can be set up as a remote head. 


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: K1DA on September 06, 2013, 07:14:47 AM
  I have a TK for use on  2 simplex (and a 100 watt version for six) and it is like a high end stereo replacing a boombox.  It's the ABSENCE of annoying burps, squeaks and buzzes that makes distant stations sound as good as those close in.   In the good olde days on ten meters tube recievers which roared and hissed sounded "hot", until radios like the S Line came along, they didn't roar and hiss but  heard things buried in the noise on the "hot" ones. 


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: KA2FIR on September 06, 2013, 08:33:54 AM
A local ham did suggest one of the Kenwood TK radios. Which model should I be looking for? I see a TK-760H on ebay. Current bid is $42. Will this work? Are they as sensitive as my TR-9130 or TM-V71A?

tnx,

Mike


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: WB5ITT on September 06, 2013, 11:00:27 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 ;D
You misread the post...the OP lives UNDER a 2m rptr....he cannot put ANY filter inline to attenuate the rptr signal enough to allow him to use a 2m simplex freq without his rcvr being desensed....A Bandpass filter has a broad bandwidth....as you found out and yes you can work SSB and FM with a BP filter in line...the OP's situation would be equivalent to YOU trying to work SSB while your FM transmitter was keyed....aint gonna work!


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: KB1LKR on September 07, 2013, 09:52:06 AM
Beyond tweaking the antenna location in search of a null and an antenna with great F/B ratio, shift to the 147.420 - 147.570 simplex freq's (15 or 20 kHz steps, as common in your state/area, likely 15) for 800-900 kHz of seperation from the 146.625 machine AND try one or more cavity filters. Expensive, but...


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: K1DA on September 08, 2013, 08:51:46 AM
   The TK 730 is the radio used for 2 meters, but there are two versions, one for the high segment, and another for the low segment which includes 2 meters.  The high "split" version can be brought down to 2, but the low split version works there  as is.  There are 50 and 100 watt versions.  The later "head unit" can control TWO TK units on two different bands at the same --i. e. a 2 meter and a 440.  You will have to do a little research on the net but there is plenty of info out there, just Google TK 730.  The radio hears very well.  Many "ham quality" vhf radios lose their ability to hear weak signals when strong signals are present on frequencies NEAR 2 meters (the VHF pager down the street)   the TK front end holds up under these conditions and that is why it makes a good repeater receiver. 


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: N8AUC on September 16, 2013, 01:36:11 PM
That's not intermod. Your receiver is getting de-sensed by the nearby strong signal.
There isn't a radio out there that will avoid being de-sensed by what is essentially an in-band signal.
And you're only 125KHz away from the repeater output. A filter capable of taking that out is waay out of my price range. I'm guessing yours, too.
The best suggestion is a directional antenna with a good notch in the pattern.
A corner reflector, pointed away from the repeater ought to do the trick.


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: W8JX on September 16, 2013, 05:16:25 PM
A dual band rig would not be first choice here. They can be pretty broad. Maybe a Kenwood TM 281 single bander


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: N3HFS on September 16, 2013, 05:27:17 PM
Some of you bring up a pretty good point -

Amateur Radio has the advantage of access to many bands, and rules that allow interplay between them.  In many places, there are linked repeaters across bands that can be used in situations such as this one. 

The O.P. might consider using a band such as 70cm as a gateway to more popular (and linked) 2m repeaters in his area.  Or, to be more precise about his dilemma, he might arrange for a frequency-agile, remotely-controlled crossband repeater at another ham's slightly distant location so that he can communicate on 2m simplex via a 70cm channel from his home.

Perhaps this isn't an ideal solution, but it is an approach that would solve the problem legally and practically.


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: WA7KGX on October 04, 2013, 04:44:34 PM
Some ideas come to mind.

Get a high level mixer and mix 146.52 down to a low frequency,
perhaps 455 KHz. 

Second, study the techniques used by C band satellite users to
avoid terrestrial interference.  Also techniques used on the low DC
bands to null out interference.   You might be able to get a 20-30db
null on the repeater's signal.

Finally, a higher gain antenna for the repeater would concentrate power
at the horizon, reducing the signal at your QTH.


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: KQ6Q on October 11, 2013, 10:04:35 PM
One thing that might work is to use a corner reflector antenna, with the null pointed at the repeater.
Arrow antennas makes a nice one if you don't have the time to homebrew.

http://www.arrowantennas.com/inst/icb148.html


Title: RE: 2 Meter simplex radio that's intermod-proof
Post by: KA9JYO on November 08, 2013, 06:28:45 AM
I use a Motorola GM 300, $60 on ebay, set on PL decode for every channel. I have no intermod, on my ham VHF I get lots of intermod.