Radio Newb, Advice on multiple antenna setup

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rwgast

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I recently purchased a a Realtek SDR dongle, and soon I would also like to get a BeoFang portable transceiver. I am an electronics hobbyist and originally picked up an SDR and MMDS down converter to help me debug my 2.4ghz xbee and nrf systems, along with analyzing wifi packets.

Since I got my SDR I am hooked on HAM activities, everything from police scanning to ADS-B plane
tracking. I would like to get in to ISS, and satlink stuff too. Currently I running a low noise amplifier and working on a Discone along with an ADSB antenna. I would like to build quite a few antennas for different purposes and mount them on my roof. I also have a nice LNA, my problem is I dont know the best way to run all these antennas in to one LNA on the roof and then in to a single feed line in to the house.

Is there an easy way to mount antennas on the roof and then switch them without climbing on to the roof to hit a switch on an a/b coax switch? The only thing I can think of is using RF relays on the roof controlled by a circuit in the house, the problem is RF relays are extremely expensive! Is there a way to switch coax lines with small signal transistors or even regular DC relays? Im looking for any solution to either switch the antennas or diplex them for a reasonable price. I plan on working from the Khz to 2.4ghz range.
 

zz0468

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...I also have a nice LNA, my problem is I dont know the best way to run all these antennas in to one LNA on the roof and then in to a single feed line in to the house.
In general, the best way is don't. Combining multiple antennas to one input is fraught with all sorts of problems, the end result being that those antennas wind up behaving less like antennas, and more just a mass of metal in the air collecting signals in random directions.

There is a sticky thread on that topic here: http://forums.radioreference.com/sp...57-running-one-scanner-multiple-antennas.html

The idea of switching them is much more viable.

Is there an easy way to mount antennas on the roof and then switch them without climbing on to the roof to hit a switch on an a/b coax switch? The only thing I can think of is using RF relays on the roof controlled by a circuit in the house, the problem is RF relays are extremely expensive!
Is there any reason why you can't run multiple runs of feedline down to the house? Let each antenna have it's own run, then you can build a patch panel, manual swap connectors, or use a relay to select. I would think coax would be cheaper than a good coaxial relay.

Is there a way to switch coax lines with small signal transistors or even regular DC relays?
Yes, but that opens up all sorts of possibilities for problems. Just stay away from the idea of transistor switching RF, unless you have the appropriate background to design it right. Modern solid state RF switching is done with PIN diodes. Building a PIN diode switch isn't excessively difficult, but I don't think it's an elegant solution to your problem.

Regular relays can switch RF at lower frequencies. There's one old trick of popping the cover off a relay, removing the wire jumpers from the relay contacts to the base pins, and connect coax directly to the contacts. Useful up to 30 MHz or so, beyond that you wind up with poor isolation, and huge impedance bumps on the line, as well as the contacts themselves acting like little antennas. There's a reason that good coaxial relays are so expensive.

Im looking for any solution to either switch the antennas or diplex them for a reasonable price. I plan on working from the Khz to 2.4ghz range.
You can spend hundreds of dollars on window filters for each antenna, combine them and run a single feedline, you can spend hundreds of dollars on a good coaxial multiport relay, and run a single feedline, or you can spend a whole lot less and run multiple feedlines, and manually swap connectors down below. Depending on where you are, and what youor good LNA is, you'll probably wind up buying filters to keep the LNA from being overloaded by local cell sites and FM broadcast stations.
 

rwgast

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The mods moved this thread a few times, kind of hard to find! Thank you guys for the advice, great thread reference.

I understand running a separate feed line to each antenna is the best solution. The problem is that I live in an apartment and Ive already knocked a hole through the wall using coax pull through grommets. Obviously your not suppose to do that, but I ran the cable neat colked it up and painted it the same coloar as the wall. If I were to use multiple feedlines to the roof I would have to either knock out more holes, or make the hole I already made bigger and install a service box outside to plug in the right antenna!

I looked around on ebay and coax relays up to a few ghz can be had for $30 or less. My idea is to add one for each antenna on the roof and then route the control wires back down with the single feed line. This way I can switch the selected antenna via a computer or micro controller. I can also run short feed lines to each relay and put my Low noise PreAmp after the relays, that way I dont need to buy an LNA for each antenna. I am using the LNA4ALL broadband LNA .7db of noise or less through 2.4ghz.

Ive seen remote antenna switchers but it seems like they only go up to about 30mhz, so im guessing those are mostly used to split the short wave bands up, has anyone seen a broadband remote coax switcher?

If you dont know the RTL SDR dongles have bad dynamic range, so this has got me wondering.. am I better off switching through different tuned antennas, or would a nice omni directional broad band antenna connected to a switchable filter box (antenna tuner) be my better bet for reciving better signle with less noise in each band?
 
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