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Author Topic: Device to repeat an entire band?  (Read 1957 times)
KK6ISN
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Posts: 3




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« on: March 23, 2018, 01:17:35 PM »

Apologies for what may be an odd question. It's not directly HAM related, but is about radio tech.

Is it possible to build a relay for an entire band? Something that had a direction antenna and RX on end connected to TX and directional antenna (pointed in a different direction) on the other end. I'd want the device to repeat everything in range, like all of 70cm or all of 33cm.

Could I just use a small amp and connect antennas to both ends? I'm not looking to add a bunch of power, but get around obstacles, especially for shorter wavelengths.

Thanks for any help.
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N8YX
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« Reply #1 on: March 23, 2018, 02:19:10 PM »

Look at the transponders which AMSAT flies on their birds.
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G7MRV
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« Reply #2 on: March 23, 2018, 02:54:04 PM »

Two highly directional/high front to back ratio antennas, with a broadband amplifier between, and sited so that the two antennas cannot each 'see' the direction of the other, is known in Television transmission as an Active Deflector. They are used to provide a relay service around an obstacle such as a hill.

Likewise, satellite transponders are typically a fairly wideband system. However, a transponder also shifts the band frequency, whereas an Active Deflector has no frequency changing.

It also depends on the 'band' to be relayed - relaying something like 30m or 17m is a simpler prospect in terms of bandwidth than relaying the entire 70cm band! But the higher VHF/UHF bands of course alow for much more compact and very highly directional antennas.
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KK6ISN
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« Reply #3 on: March 23, 2018, 03:05:14 PM »

Thanks for the replies!

So if I wanted to build something, it seems like wide band amp, with two directional antennas and antenna placement so the output antenna doesn't interfere with the input side as much as possible.

Thanks!
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KL7CW
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« Reply #4 on: March 23, 2018, 08:59:57 PM »

First of all in most cases you will need an active repeater since only a small percentage of the transmitted power from the shack will be received by the remote antenna.  In the past I have worked out calculations and passive (reasonable size) repeaters usually only work out if the remote dish is very close to one end of the circuits.  As G7MRV says, often the preferred method is to have the shack to remote repeater on a different band or frequency.  Often a microwave or UHF link is best.  If you must use the same frequency, different polarization for the shack to remote repeater can easily add more than 20 dB of additional isolation.  Commercial microwave relay stations use very directional antennas, and do not reuse the same frequency for at least several hops and also design the system so when the frequency is reused the antenna of the origination station is not lined up with the relay station that reuses this frequency perhaps more than a hundred miles away.  Please research this carefully since there are probably FCC restrictions on this topic.  Possibly you could use an optical link Huh
              KL7CW
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KG4RUL
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« Reply #5 on: March 24, 2018, 07:20:51 AM »

Since we don't know what frequency band or class of service you are referring to, I will make the blanket admonishment to observe any power and rf exposure limits or transmitter licensing and identification requirements that are imposed for that service.
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G3RZP
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« Reply #6 on: March 24, 2018, 07:36:01 AM »

A similar application is the 'Serrodyne', used in electronic warfare, except that the Serrodyne adds phase modulation, so that a radar can be 'spoofed' into thinking a target is approaching it or, alternatively,  going away because of the Doppler shift impressed on the amplified returned signal.
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KK6ISN
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« Reply #7 on: March 24, 2018, 11:20:51 AM »

What I'm thinking is the "active deflector" concept described earlier,  built from an amp that covers the range I'm interested with two directional antennas, and doing different polarity on the input side as the output side. The thing I'm trying to do is analog video for an R/C aircraft around an obstacle. The power output of the amp will be no more than 500mW. I want the gain on the antennas to do the work, not the power of the transmitter. I'm looking at using  900Mhz, 1.3Ghz, or 5.8Ghz. Not all of these bands at once, of course. I haven't yet settled on which I will use end-to-end.

A question about identification requirements. The video TX is a station, and I have it set to identify periodically. Does this active deflector count as another station that needs to do its own identification as well?  My mental model for this is that the deflector is an amp I'm using to establish a link between between a video TX and a RX which are not LOS to each other. The active deflector isn't tuning to a channel, modulating a signal or anything. It is picking up whatever it hears on the input side adding power and sending it out the output side, relying on the antenna and amp to attenuate frequencies outside the band they're designed for.

I just re-read the above paragraph and see how it could be taken that I'm arguing. I do not mean it that way. I want to understand the requirements and improve my mental model for what is required. Thanks for continuing to help.



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