Difference between revisions of "Building an RF Receiving Station"

Revision as of 16:46, 14 March 2019

Myself and other friends enjoy using scanners and now SDR's to find interesting radio signals to monitor. At Bloominglabs we have a receiving station we've been continuing to work on and improve for a while. It's been a fun and rewarding project which we keep finding new things to expand with. The first part of scanning involves getting an antenna in the air. Then it's a matter of getting it connected to your gear. There are a lot of components to making this work correctly. This project is open ended and experimental so we need to be able to split the signal between a number of receivers.

Antennas

I bought a generic wide-band discone antenna several years ago for Bloominglabs to hook up to a radio scanner. Discones are good for receiving because they are omni-directional and they can cover many frequencies. They can also be used for transmitting but they are not as optimal for this as a proper tuned antenna for the band you are transmitting on. We moved this antenna from our last building to our current building as of 2018. Obviously height is an important factor, ours is on a 10' mast mounted to the side of our building. Our elevation relative to surrounding buildings is not great, but with the mast raised we get reasonable reception of a fair amount of spectrum.

Connectors

RF connectors are mainly selected for 2 criteria: RF losses in dB and weather resistance. Losses are always more important the higher the frequency you go. If you want to work with RF much over 500MHz then you want very low loss connectors and coax. Most discone antennas have UHF or SO-239 (female) connectors on them which mate with PL-259 (male) connectors. UHF connectors have reasonable performance for VHF and UHF frequencies but N-connectors are much lower loss and much more moisture. N-connectors are just about always found on S-band (2.31 to 2.36 GHz) and higher rated gear because they inject the lowest losses. You can use N-connectors for any system and in fact it's a good idea if possible, they're the best of both criteria. However UHF is more common, especially on discone antennas. Some scanners and equipment use BNC and other lower quality connectors too.

For our purposes I started using SMA connectors for as much as possible which are good up to 18GHz. Most smaller RF equipment like handheld radios and SDRs use SMA already. They are reasonably good at RF noise rejection, have low losses, and are cheaper than N-connectors. They have a side-benefit of being physically smaller which has other benefits when selecting a splitter. Bought in bulk they can be had for just a couple dollars a piece.

A lot of RF gear also uses BNC, F-connectors, and other less common connectors. I've used a number of these over the years along with adapters. With the main feed and splitter for our receiving station we're tried to avoid anything but UHF, N, and SMA connectors. Once the feedline reaches the distribution point it's less critical, but using as low as possible loss connections to that point improves signal quality for all final devices connected.

Coax

stuff and words

Budget coax jumpers and crimp tools

It does take some practice but if you want to be as budget minded as possible then making your own coax jumpers is the only way to go. Crimpers sold for RF use are often expensive, but you can get them cheaper if you know what you are looking for. You want a standard ratcheting crimper which will last a long time, avoid single size crimpers without replaceable dies as they are usually weaker and may not fully crimp, and the tools can wear out pretty fast. Also, look at the cost of a crimper and die set spread across the cost of all the connectors you plan to use it for versus buying your coax jumpers pre-made. If you need more than just a few jumpers you can come out ahead. And being able to fix bad connectors on used coax and being able to make jumpers exactly to size is a nice benefit.

Several companies sell compatible ratcheting crimper tools like these which can have the dies replaced:

• Voilamart Insulated Wire Crimper - $12 via Prime, same tool other companies sell for over $100 elsewhere

Paladin, Datashark, and Greenlee crimpers are also compatible with the same dies. Bigbox stores usually carry these crimpers from $30-$40. The tools all have a ratcheting action and a similar look with the slightly off-set head. There is a star washer and screw adjustment on the side of most of these crimpers which allows you to set the depth of the crimp. I don't think I've ever had to mess with this.

Now you need dies for the size of coax you will be crimping. These dies include crimps for both the external shell and the center conductor. They are all compatible with the above crimping tools:

• Crimp dies for LMR240/RG-8X - $25 plus shipping
• Crimp dies for LMR400 - $20 plus shipping

Obviously you will need to remove whatever original die set was in the budget crimper you found and replace it with the above dies. We needed both sets in order to be able to make our own LMR400 ends as well as RG-8x jumpers. I recommend soldering the center conductor lug on though rather than crimping as I've found my crimps on these to not be 100% reliable. The outer shell can be crimped though of course.

For coax strippers I highly recommend a trusty pocket knife and nothing more. Any cheap razor-blade based tool I've ever used never can cut the outer shield without nicking something inside. With a little practice you can get this right the first time every time. Another important note is that the metal sheath of the metal connector should only go under the coax shield braid, it should not slip under the outer sheath at all, the outside crimp ring will not fit otherwise.

Moisture and coax

Moisture is the enemy of coax, it will slowly degrade outdoor runs over time. Moisture gets into coax through the connectors, for outdoor runs its good to wrap the mated connector set with tar cable wrap. Its a little messy but very worth it. I've not had success with the silicone-based wrap tapes. It's possible I just don't know how to use those right, but I've tried several times and failed each time to get the tape to lock in place.

Multicouplers/Splitters

Obviously we want to hook up a lot of gear to this antenna setup. Splitters are expensive, but if you know a little more about them you can get some excellent deals on better equipment than usual scanner splitters. Most people know about this unit here:

• 4 Port MCA204M VHF UHF Receiver Multicoupler - $200 new

Notice that it uses BNC connectors, and it's expensive. With a little bit of knowledge you can easily dig up much better splitters on Ebay at a fraction of the cost of the above unit.

Power Dividers

Splitters and multicouplers are actually called power dividers by RF engineers. An ideal power divider perfectly splits (or combines) the input signal between all of its output ports. It should also electrically isolate each port so even a short on one output will not reach any of the other ports. Last but far from least, it should work for the entire band of RF spectrum you intend to receive. In our case we want everything under 1GHz to be available since we're just experimenting and don't know what all we intend to receive. You also need to consider the connectors the power divider uses, in our case I wanted SMA connectors which simplifies the number of connectors I need to carry for each coax type.

So, knowing some new terms now, if you go to Ebay and search for a power divider you will get a ton of surplus government equipment. It's very important that the divider works for the RF band you need to cover. Most dividers are for microwave equipment and don't go below .5GHz (500MHz). However the equipment does exist if you are persistent. Find a divider with the number of ports you want and then verify it works for your RF band. For 4 and under ports there are usually lots of dividers that work from 50MHz to 1000MHz. This is a wide-band power divider. Also, on the surplus market these often go for less than $50.

What about "combiners"? These work just fine as a divider, and in fact electrically they are the same. Also, most dividers will have a power rating in Watts and a VSWR measurement. You don't need to worry about these ratings since you are only receiving with it.

Surge Suppressors

If you are hooking a bunch of gear up to an outdoor antenna then you will need to protect it. "Lightning suppressors" aka surge suppressors will help. Most folks go with Diamond for this as they are the cheapest out there. These are what we use, as far as we know they are reliable suppressors. If money is no object then go with Polyphasor, but that's not in our budget.

• Diamond SP1000 - UHF su-pressor
• Diamond SP3000 - 2.4GHz suppressor

The SP3000 uses N-connectors so we went with that to get the better RF performance and lower losses for the entire system. The SP1000 uses UHF connectors. These are both indoor devices, Diamond also makes outdoor units too. Just be sure to note that the outdoor units have both male and female connectors on them (so they can attach directly to the antenna ahead of the feedline). The indoor units have female connectors on both ends which is what you want when connecting to a grounding location with a short pigtail going to your radio equipment.

Also note that direct hits by lightning are not guaranteed to be protected against. However they should protect you against static surges and near hits. Obviously you want the suppressor between the splitter and the antenna.

Grounding the system

I recommend an 8' grounding rod to ground your antenna mast. You will need some steel cable or similar to connect from the ground rod to your mast. You also need a good ground from your indoor surge suppressor out to your grounding rod. Most big box hardware stores have bronze grounding clamps for various size pipes along with attachments for grounding rods. There are other schools of thought, but for a simple antenna mast I prefer to prevent static electricity buildup with the intent of preventing a lightning strike.