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Author Topic: Solar-powered, Portable 2 Meter Repeater Inquiry  (Read 25639 times)
5R8Z
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Posts: 11




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« on: October 06, 2012, 03:52:27 AM »

There are 22 licensed hams in Madagascar according to OMERT. I am unsure how many speak English. I have only spoken with one other 5R8- license here on the island on 20M.

I live in the rainforest of Madagascar and the mountainous terrain kills VHF quick. There are no repeaters, igates, etc here that I know of, particularly my area. There is zero VHF activity near except for the birds passing over. I want to build a solar-powered, portable 2 Meter Repeater from materials I have on-hand to provide local comms with my family. I have used an Argent Data Simplex Repeater but want to ditch the record-then-repeat delay. Trying to have a 10 minute conversation takes 2.5x as long with all the record-then-repeat delays and the doubles.

I have:

two 2 Meter HTs
one 2 Meter 50W mobile
one FT-897d with AT-897
one Kenwood TS-50 with Kenwood ATU

FROM WHAT I HAVE ON HAND, (no Amatuer Radio supply stores here) I want to use one HT (with its own antenna) as receive on frequency 14X.XXXMhz and send the HT audio output to the input on the 2 Meter 50W mobile radio (with its own antenna). There are two antennas in this scenario.

Keeping in mind buying duplexers and various other conveniences is not an option and that the goal is to make functional repeater with items on-hand for only two people to use:

Will using a larger shift than +/- 600khz help the interference between the HT and the 50W mobile?

Ignoring the solar power components (panels, battery, charge controllers, low-volt disconnect, fusing, etc.), does anyone have suggestions that can help improve the setup to eliminate interference?

thanks for any advice,

Tim
5R8Z in Madagascar
KJ4NCD in USA





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N4CR
Member

Posts: 1666




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« Reply #1 on: October 06, 2012, 02:43:20 PM »

HT's have notoriously bad receive filtering for strong same band transmitters. It's going to be really easy to overwhelm it's input and cause fundamental overload with the transmitter.

That's probably the worst obstacle you'll meet when setting up with what you have on hand.

Vertical separation of the antennas will help. You might be able to make a coax stub notch filter for the RX coax on the transmit frequency that will help some. It won't be easy to tune without any equipment but eventually you'd find the trap frequency by trial and error.

Wider frequency separation will help but you'll find that fundamental overload won't change much with how far the frequencies are apart because fundamental overload causes desensing via AGC and that usually happens before the tuned stages.

The other thing you'll face is that consumer equipment is really made for about 25% duty cycle. Repeater operation generally approaches 100% duty cycle for the time of your entire conversation. You'll want to turn the power way down on that mobile transmitter in order for it to survive 100% duty cycle.
« Last Edit: October 06, 2012, 02:48:53 PM by N4CR » Logged

73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
N4CR
Member

Posts: 1666




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« Reply #2 on: October 06, 2012, 03:04:40 PM »

Sounds like a fun project with plenty of room for experimentation.

Here's some information on how to make a tuned coax stub trap...

http://ac0c.com/main/page_so2r_coax_stub_intro.html

This is for HF stuff, but the principles don't change other than making the tuning process more difficult because you are working with smaller measurements.

I've seen coax stub traps with a cap at the end instead of a short so you can tune the stub with the variable cap. That would probably help but it could introduce some variables that would be very hard to take into account in the field without test equipment. Much more experimentation if you choose to do this.

Also, if you have both rigs connected to the same power supply, you might get RF coupling that way. You'll need some chokes in the wiring to stop that. You can home brew low power chokes with wire wrapped on an iron core with a cap to ground in the power supply side. You might have to put a choke in both power leads.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
N4CR
Member

Posts: 1666




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« Reply #3 on: October 06, 2012, 03:11:43 PM »

Actually, since you can be frequency agile, you could make the stub, figure out where it's tuned with experimentation and then put your transmitter on that frequency.

That's something you can't do when there's a band plan with hundreds of other repeaters in the area. Seems like it would be a lot easier than trying to hit some target frequency with the stub building process.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
AD4U
Member

Posts: 2164




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« Reply #4 on: October 07, 2012, 07:22:59 AM »

How about a remote cross-band repeater.  It only requires one rig and it does not require a duplexer (very expensive) and is easy to set up in 10 minutes or less.  445 MHz input and 146 MHz output.  I realize this is not an ideal solution but it may get you by until you come up with a "real" repeater.

Dick  AD4U
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5R8Z
Member

Posts: 11




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« Reply #5 on: October 07, 2012, 12:07:19 PM »

N4CR and AD4U,

thanks for the info and links. I will need about a week to chew on this before I can comment further.

As for the cross-band repeater, I only have two 2 Meter HTs and a 2 Meter mobile, no 440 HTs or mobile. That may be something I could purchase next year when in the States to complete the final two ham licenses.

tim
5R8Z
KJ4NCD
« Last Edit: October 07, 2012, 12:50:22 PM by 5R8Z » Logged
5R8Z
Member

Posts: 11




Ignore
« Reply #6 on: October 08, 2012, 02:18:27 AM »

HT's have notoriously bad receive filtering for strong same band transmitters. It's going to be really easy to overwhelm it's input and cause fundamental overload with the transmitter.

That's probably the worst obstacle you'll meet when setting up with what you have on hand.

Vertical separation of the antennas will help. You might be able to make a coax stub notch filter for the RX coax on the transmit frequency that will help some. It won't be easy to tune without any equipment but eventually you'd find the trap frequency by trial and error.

Wider frequency separation will help but you'll find that fundamental overload won't change much with how far the frequencies are apart because fundamental overload causes desensing via AGC and that usually happens before the tuned stages.

The other thing you'll face is that consumer equipment is really made for about 25% duty cycle. Repeater operation generally approaches 100% duty cycle for the time of your entire conversation. You'll want to turn the power way down on that mobile transmitter in order for it to survive 100% duty cycle.

All of this is very helpful, thank you.

For the coax stub notch filter I have RG-231 and RG-8x. Do you suggest one over the other?

In addition to the coax stub notch filter, would placing the rx HT in one steel or aluminum container and the tx Mobile in a different steel or aluminum container aid in shielding? I have seen galvanized sheets available in a town about 1.5 hours away that could be crafted into individual containers if this would help reduce interference.

thank you for input,

tim
5R8Z
« Last Edit: October 08, 2012, 02:22:26 AM by 5R8Z » Logged
W9IQ
Member

Posts: 102




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« Reply #7 on: October 09, 2012, 03:36:51 AM »

Hi Tim,

Here is some more information for you related to the antenna portion of your project. For vertical dipoles on 2 meters, here are some working numbers that may help you relative to antenna separation:

Horizontal Separation

 30 ft - 30 dB
 50 ft - 35 dB
100 ft - 40 db

Vertical Separation

 5 ft - 20 dB
10 ft - 35 dB
30 ft - 55 dB

Combining horizontal and vertical separation allows you to practically add the dB's of each together.

The working rule of thumb for a 100 watt repeater on two meters is to achieve > 85 dB of isolation when operating at a 600 kHz spacing or >75 dB when operating at a 2 MHz spacing. Your requirements will be less than this since presumably the HT you will use as the transmitter will be putting out <5 watts. This is tempered, however, by the poorer qualities of the receive HT compared to a commercial grade receiver.

There are some other practical considerations:

1.) Coax cables that are not double shielded will substantially reduce the antenna isolation numbers if the transmit and receive cables are in near proximity to one another. Keep them as isolated from one another as possible.

2.) Keeping the two HT's physically separated will help to improve the system performance. Consider mounting each HT very near the respective antennas and run power and audio with well filtered cables. You may also wish to put each HT into some type of grounded, metal container for additional filtering protection.

3.) Make certain to include several baluns along the length of each antenna cable. A couple of 6-8 inch turns of the coax every so often will help eliminate common mode currents. These can cause potential antenna pattern alteration and source of additional receiver desense.


Good luck with the project!

- Glenn DJ0IQ and W9IQ
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WB6BYU
Member

Posts: 13230




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« Reply #8 on: October 10, 2012, 11:07:20 AM »

Quote from: W9IQ

Combining horizontal and vertical separation allows you to practically add the dB's of each together.



Unfortunately it doesn't work that way.

Vertical separation works because of the nulls off the ends of the antennas.  If the
antennas are also separated horizontally they are no longer in the nulls, and only
the horizontal separation provides isolation.

My recommendations would be:

1) use the HT for transmitting and the mobile rig for receive.  That reduces your
isolation requirements due both to the lower transmit power and the fact that the
mobile rig probably is less susceptible to desense.  If you do use the mobile rig to
transmit, use the lowest practical power setting.  If you are using an HT to talk
to the repeater, there isn't a lot of advantage of having it run 50 watts output.

2) isolation is improved if the ENTIRE transmitter and receiver are separated as
much as possible, because neither is well shielded as is, and even putting them in
fully shielded boxes you still have leakages from the cables, power leads, etc.
So separate the rigs by 100 feet or more and bury the power and audio leads
between them to reduce the RF via that path.  The ideal situation might be two
trees about 300' apart along the crest of a ridge, using one for the transmit
antenna and one for receive.  With such spacing there will be directions where
the repeater transmits better than it receives or vice versa, but it makes
a more practical system when you have limited capabilities for isolation by
other means.
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5R8Z
Member

Posts: 11




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« Reply #9 on: October 12, 2012, 12:01:40 AM »

Thanks to each of you for your advice. Due to the materials on hand, I will use two vertically separated antennas to keep the one solar panel and battery near to the rx HT and tx Mobile. I think too much horizontal separation may tax the solar panel and battery power supply. I may also seek to house the HT and Mobile in separate shielding containers that I can ground along with the panel and battery. However, the final build needs to be easily deployed or moved which weighs in on design. In the absence of analyzers or in-field meters, live testing may prove to be the only way I can make final adjustments to minimize interference. I have considered using vertically oriented dipoles, one tuned for rx and one for tx. I only have a little time each week to give to this project and continue to use the Argent Data record and repeat in the interim.

tim
5R8Z
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5R8Z
Member

Posts: 11




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« Reply #10 on: October 14, 2012, 08:57:44 AM »

We have a Category 3 Cyclone headed towards us so the repeater project is on hold for now. You can see the cyclone's path here:

http://www.wunderground.com/tropical/tracking/si201201.html
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N4CR
Member

Posts: 1666




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« Reply #11 on: October 18, 2012, 03:03:49 PM »

We have a Category 3 Cyclone headed towards us so the repeater project is on hold for now. You can see the cyclone's path here:

http://www.wunderground.com/tropical/tracking/si201201.html

The store seems to be just hanging there. Either that or the Sat pictures aren't being updated.

I hope you fare well through the storm.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
W9IQ
Member

Posts: 102




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« Reply #12 on: October 19, 2012, 06:53:39 AM »

Hi Tim,

I hope you are holding out OK in the weather.

I had another thought about your repeater project. When I was doing research for an article about duplexer designs, I came across several web posting from people who had made 2M duplexers out of metal barrels or metal buckets. Clearly far from ideal but when working on a rubber band and binding twine type of project, it might give you the edge you need. You might want to check them out.

- Glenn DJ0IQ and W9IQ
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W9IQ
Member

Posts: 102




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« Reply #13 on: October 19, 2012, 09:07:46 AM »

I neglected to respond to WB6BYU's comment regarding my assertion regarding the combination of horizontal and vertical separation figures. He is absolutely right, the combination of the two is more complex than the simple addition I had suggested. Thanks for catching my error, Dale.
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5R8Z
Member

Posts: 11




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« Reply #14 on: October 22, 2012, 06:08:52 AM »

The storm hit category 3 and then died out. We received rain on Saturday to fill the cistern a little. At least we were prepared with food for the kids and fuel for the genny. I've got a lot of projects to close down before I can play radio for a while. The duplexer idea from W9IQ about using metal barrels or buckets sounds wild and hope to research that soon, but certainly sounds "anti-portable." Any site the repeater will be deployed to will be hiked up; no road accessible mountaintops here.

tim, 5R8Z
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