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Author Topic: Add FM to a boat anchor?  (Read 1872 times)
KX4QP
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Posts: 404




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« on: May 02, 2019, 04:19:14 PM »

I'm still trying to get into my club's weekly nets; none of their repeaters are reachable from my home (at least with an 8W HT).  I was holding out hope for the one 10m repeater that connects -- then realized that my SB-102, which will work 10m, is CW or SSB, while repeaters are FM (even on 10m). 

That leads me to wonder if there's any way to add FM capability to the 10m band in my SB-102 (once I have it fully functioning)?  Seems to me I'd have to pull the LMO output, run it through the appropriate modulation process in place of the SSB mixer and filter system, and put it back in at the PA.  Doesn't sound impossible, though fitting it inside the housing and putting usable controls on the front might be tricky (there are already a lot of knobs on the front).  At least I could reasonably expect 10m to reach thirty-five miles to that repeater with 50W+ output, even with a couple hills in the way.
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VE7DQ
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« Reply #1 on: May 03, 2019, 01:27:19 AM »

Anything is possible; whether it's practical or not, is another story.

Beside adding FM function to the SB-102 in both transmit and receive,  it is necessary to have a substantial offset between transmit and receive frequencies to work repeaters (plus a subaudible tone), and that will be a challenge to implement without considerable research, effort, time... and money.

What I'm trying to say is... it ain't practical.  No way.  A fully-functioning SB-102 shouldn't be butchered.

You didn't mention what band the club's repeaters are on, and I know nothing about your circumstances or environment, but if you are not using an outside antenna on your HT, I suggest that you explore that option.  Even a 3-element beam pushed up on a mast would improve your odds.  More elements are better.  Perhaps something other than an HT (assuming 2M or 440) would be another approach (with your external antenna).
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AC2EU
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« Reply #2 on: May 03, 2019, 06:40:45 AM »

I would check the "impractical" box on that one.
However, you have a HT and it's Hamfest season, so there will be lots of inexpensive 2 meter amps on the tables.
All you need to do is adapt the HT antenna connector to a cable with a PL239.
A better antenna might help also.

Here, I only have to adapt the HT to my outside 2M vertical with elevated ground plane and it can key just about any repeater within reasonable distance.
Elevation has a lot to do with it as well...
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AA4PB
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Posts: 15066




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« Reply #3 on: May 03, 2019, 07:22:43 AM »

Assuming that you are using the HT with its rubber duck antenna, the best thing you could do is to put up an outside antenna. A vertically mounted Yagi at 35 feet can make a huge difference. At one time I had an 11 element Yagi at 35 feet and was able to reliably reach a 2M repeater that was about 100 miles away.
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Bob  AA4PB
Garrisonville, VA
N8YX
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Posts: 1396




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« Reply #4 on: May 03, 2019, 09:40:36 AM »

There are a ton of early-generation solid-state ham rigs that'll do 10M FM...cheap.

Or find a low-band Syntor and have it programmed for both the simplex channels and the repeaters. Again, cheap.
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AC7CW
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Posts: 1356




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« Reply #5 on: May 03, 2019, 01:35:13 PM »

Get the SB-640 matching VFO and you can operate split. That much is doable... adding in FM modulation and demodulation is perhaps an interesting project but not one that I would want to undertake nor recommend.
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Novice 1958, 20WPM Extra now... (and get off my lawn)
G3RZP
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« Reply #6 on: May 03, 2019, 04:33:14 PM »

Some older boatanchors on Am on 10 had enough incidental FM you didn't need to do anything!

But the SB102 isn't one of them. Easier to build a  10m FM rig.......
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KX4QP
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Posts: 404




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« Reply #7 on: May 03, 2019, 05:26:03 PM »

Here, I only have to adapt the HT to my outside 2M vertical with elevated ground plane and it can key just about any repeater within reasonable distance.
Elevation has a lot to do with it as well...

Well, I didn't think it would be a practical mod, especially given my level of (in)experience with hacking on boat anchors -- but I was kind of hoping I'd get something like "oh, you just need to find an XYZ module for that Heathkit; they're not common, but they turn up at hamfests every so often."

Sadly, elevation is one thing I don't have.  My house is roundly 60 feet below the average of surrounding terrain; the one club-connected repeater I might reasonably reach (at 30 miles, but up a thousand feet or so in a pass) has roundly a thousand feet of a hogback ridge sticking up above the sightline between me and it -- and none of the repeaters located on said ridge (one of which I use regularly) are connected to the club net (nor are the operators friendly to the idea of changing their setup).  Even a yagi on a mast (and I can't afford a mast any time soon) won't reach through a couple hundred feet of hilltops between me and all the other repeaters I'd need to key.

I doubt even a 50W base rig with a yagi that looks like a giant's nit comb will punch through the terrain I'm up against.  Won't be a problem when I have my HF antenna up, 80m and 40m will just diffract over the terrain -- but 2m (never mind 440) isn't going to treat a hilltop as a knife edge.
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AC2EU
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« Reply #8 on: May 04, 2019, 08:11:45 AM »

Here, I only have to adapt the HT to my outside 2M vertical with elevated ground plane and it can key just about any repeater within reasonable distance.
Elevation has a lot to do with it as well...


Sadly, elevation is one thing I don't have.  My house is roundly 60 feet below the average of surrounding terrain; the one club-connected repeater I might reasonably reach (at 30 miles, but up a thousand feet or so in a pass) has roundly a thousand feet of a hogback ridge sticking up above the sightline between me and it -- and none of the repeaters located on said ridge (one of which I use regularly) are connected to the club net (nor are the operators friendly to the idea of changing their setup).  Even a yagi on a mast (and I can't afford a mast any time soon) won't reach through a couple hundred feet of hilltops between me and all the other repeaters I'd need to key.

I doubt even a 50W base rig with a yagi that looks like a giant's nit comb will punch through the terrain I'm up against.  Won't be a problem when I have my HF antenna up, 80m and 40m will just diffract over the terrain -- but 2m (never mind 440) isn't going to treat a hilltop as a knife edge.

Yes that sounds like a difficult situation for VHF.
While line of sight is the preferred method of propagation, VHF has the advantage that it can also be "bounced" off of objects.
If you had a Yagi (or could borrow one) , it might be fun to try reflecting off of other terrain to "get around' the high ridge if there is a break somewhere.
It's playing pool geometry with waves!
There also the" knife edge diffraction" phenomena that might work.

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KX4QP
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Posts: 404




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« Reply #9 on: May 06, 2019, 03:22:26 PM »

I keep hearing about this "knife edge diffraction."  To quote Inigo Montoya, "I don't think that word means what you think it means."  First, the top of a hill (even a hogback ridge) isn't anything I'd call a knife edge, even relative to 2m radio waves, and second, geometrically, I'd have to be able to diffract downward roughly fifteen degrees with enough power remaining to key the repeater -- and the repeater would have to do the same for me to hear it!

There are two other landforms high enough and in roughly the right direction to try reflection to get around Sauratown Mountain and reach the repeater in Fancy Gap -- Pilot Knob and Hanging Rock.  Pilot Knob is mostly a rounded butte, with the titular knob sticking up about fifty feet.  Not enough surface or face there to reflect from, I don't think, and if I did get a bounce off the knob itself, it'd be aimed too high and the repeater would catch only scatter.  Hanging Rock, on the other hand, is a pretty gradual slope on the side that might possibly reflect in the right direction.  This is something that seems to me more likely to work with buildings than with landforms.

I've thought about building a Yagi -- though I was actually thinking more of making it for 440 band, since that gives an antenna hardly bigger than a modern satellite TV dish.

I've also heard of "passive repeaters" -- essentially just a half wavelength conductor, correctly oriented for the wave polarization, that reradiates at around -20 dB relative to the signal aimed at it.  For 440, that would only require sneaking a few 13-14 inch wires up to the top of Sauratown Mountain and finding them with a fairly tight beam.  The down side is, it's all private land up there, the only place to put a wire like that without it being noticeable would be in a tree, and trees do a fine job of blocking 70cm radio.
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KB4QAA
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« Reply #10 on: May 08, 2019, 04:01:37 PM »

"First, the top of a hill (even a hogback ridge) isn't anything I'd call a knife edge, even relative to 2m radio waves, and second, geometrically, I'd have to be able to diffract downward roughly fifteen degrees with enough power remaining to key the repeater -- and the repeater would have to do the same for me to hear it!"

Well, I"m not sure YOU understand what knife edge diffraction truly is.  It does not require a peak sharp enough to shave with. And YES, the diffraction does allow signals to radiate downwards on the far side of the ridge.

For 2m work, I believe a yagi antenna is the best likely solution.  Get one of your club members to come over with a yagi and see is you can hit the repeater...

For 10m FM repeater work, get one of the many 1980's HF radios that are FM split capable. They are now widely available well under the $500 mark.  Best to buy one from a local ham who can demonstrate that it is in working order.
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KA0HCP, ex-KB4QAA Relocated to Ks. April 2019.
N8YX
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Posts: 1396




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« Reply #11 on: May 09, 2019, 06:15:30 AM »

For 10m FM repeater work, get one of the many 1980's HF radios that are FM split capable. They are now widely available well under the $500 mark.  Best to buy one from a local ham who can demonstrate that it is in working order.
TS-440/940, IC-745/751A, FT-980/FT-901, 902DM would be on my short list. As would a Ten-Tec Omni V/VI or Paragon II. For that matter an HR-2600 into a decent antenna will work sufficiently, and it'll do splits "out of the box". This particular radio also features a tone encoder for repeaters which use PL.
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W1BR
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« Reply #12 on: May 09, 2019, 12:14:55 PM »

Good point.  The FM board was no longer an option in the Ten Tec Omni VI series.  Used 563 Ten Tecs can be had for cheap these days.
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KD0REQ
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Posts: 2404




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« Reply #13 on: May 09, 2019, 02:56:23 PM »

theoretically, you could warble the VFO by modulating a varactor connected to the tuning capacitor. won't do anything for receive, though. and the crystal filter won't help you much. practically... nah.

can't hurt to build a 2 element "plumbers special" yagi f and see if you can bounce off something to hit the repeater. copper pipe on a varnished 2x2 is good enough to test with. try that with the HT. it's only time you're investing.
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KX4QP
Member

Posts: 404




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« Reply #14 on: May 09, 2019, 03:22:38 PM »

can't hurt to build a 2 element "plumbers special" yagi f and see if you can bounce off something to hit the repeater. copper pipe on a varnished 2x2 is good enough to test with. try that with the HT. it's only time you're investing.

Well, and the cost of some heavy copper or small tubing, and a piece of coax, plus a suitable connector.  No, not a bunch of money, but not free for someone who's been in the hobby a couple months and doesn't have a junk box (and whose partner would object to the very idea of storing "junk" just because I might use it someday).  I was actually considering building a Yagi (5-6 elements) with EMT elements and PVC spine, but tabled the idea when I found out how much earth is in the way.

Am I correct in understanding even a many-element Yagi doesn't produce a beam so narrow you'd need something like a rifle scope to aim it?  You get several degrees from anything that isn't a dish, right?

And @KB4QAA -- no, I really don't "understand how knife edge diffraction works," apparently.  Or else it's badly misnamed.  I had understood that it was a phenomenon engendered by the sharp (relative to wavelength) termination of an opaque boundary.  In terms of visible light (where it's mainly discussed in terms of photographic optics), it doesn't become an issue until your aperture is very small relative to your focal or cast distance; f/32 or f/45 start to show loss of sharpness to diffraction, in most cases.  In pinhole cameras, diffraction is always an issue; it's the reason why, no matter how patient you are (to wait for longer and longer exposures), making the hole smaller often makes the image quality worse instead of better as you'd expect if light weren't waves.

On that basis, with a wavelength of two meters, I wouldn't expect an edge with a radius larger than a couple dozen meters to have any significant effect.  It sounds like you're saying I might expect significant diffraction from something with a radius two or three times that.
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