RF cable

[timtheenchanteruk]

I have just got some new radios, and need to make up some RF patch leads for the installed RF at my local theatre.

 

the 2 cables that come with the system are URM43, with TNCs on the end for the rack/antenna.

 

I need to make up 4 TNC to N type

 

the lengths involved wont make a difference I dont think, 2 very short, about 0.3m 2 about 2m.

 

If I were buying the cable I would go straight for the URM43, however I already have about 50m or RG58.

 

from what I gather (well, know) they are both 50ohm cables, I believe that the RG58 will be fine, just want to check before spending time on something that will be unreliable as I can't find any info on the URM43 cable spec to compare.

[Stan Hope-Streeter]

<br />I have just got some new radios, and need to make up some RF patch leads for the installed RF at my local theatre.<br /><br />the 2 cables that come with the system are URM43, with TNCs on the end for the rack/antenna.<br /><br />I need to make up 4 TNC to N type <br /><br />the lengths involved wont make a difference I dont think, 2 very short, about 0.3m 2 about 2m.<br /><br />If I were buying the cable I would go straight for the URM43, however I already have about 50m or RG58.<br /><br />from what I gather (well, know) they are both 50ohm cables, I believe that the RG58 will be fine, just want to check before spending time on something that will be unreliable as I can't find any info on the URM43 cable spec to compare.<br />

<br /><br /><br />

 

RG numbers are based on USA military specs, whereas UniRadio numbers are based on British mil specs.

 

RG58 should be fine. Both the cables are 50 ohms and have similar losses. Quite high at UHF, but you wouldn't get RG213 or 214 into a TNC anyway.

 

The only thing to beware of is if you're using crimp tooling, you will need to check the specification of the crimpers to see if the cable you are using is in their compatibility list.

[timtheenchanteruk]

cheers, I have mast head amps, so that should counterbalance any loss, its supposed to according to the spec of them anyway work up to 30m cables, and the entire run including the installed cable isn't that long.

[Jivemaster]

With leads that short losses due to badly made connections could be most significant, so spend some time and effort getting the best connectors, meaning ones that connect to the cables best with the tools that you have or can borrow.

[cedd]

Hi Tim

To paint a picture for other members when replying to this thread - the theatre in question (assuming it's Yeadon Tim) has 2 lengths of coax installed within the fabric of the building, from the FOH position out to the sides of the balcony, where they appear on faceplates with N type connectors. Myself and another engineer who uses the vene a lot did the installation. It was done because we were pulling other cables and we were fed up of using cable ties every time to run our aerials out. Now we each just have a set of N type fly leads to our radio mic racks - very easy to set up.

 

So, the cablng hidden behind all the balcony covers is RG213 - the fat stuff! It's also a very high grade cable which was left over from a job at my work. You won't get much better quality coax without going for heliax, in which case you're welcome to terminate it cause I haven't got a week (it's a dark art!).

 

My own fly leads for the venue are also RG213, simply cause I had it lying around. There are some quite considerable differences in attenuation of G58 versus RG213. 58 has 20.1dB per 100f at 900Mhz, whereas 213 has 7.7dB's. Personal preference would be to RG213 just because whillst it is harder to coil, I've never seen kink damage on 213, whereas I've seen lots on 58. One sharp kink when coiling it seems to split the outer insulation pretty easily. I can also terminate RG213 into N types in my sleep, it seems to be all I do at work at the minute! Also, having made quite a decent job of the installation, it sems a little bit counter productive to use a more lossy cable for the last stretch. I use N types at both ends, as my radio mic rack has N type inputs on a rack plate.

I realise you need to terminate into TNC's which may well rule out direct connection to RG213, but if you use an adapter or as I've done, a rack plate, I means you get a less lossy and more robust cable assembly.

[mackerr]

My own fly leads for the venue are also RG213, simply cause I had it lying around. There are some quite considerable differences in attenuation of G58 versus RG213. 58 has 20.1dB per 100f at 900Mhz, whereas 213 has 7.7dB's.

 

While RG213 is much better than RG58 for long runs, LMR-400 is better yet at 3.9dB/100' at 900MHz, and in situations where you don't have to wrap it up every night, LMR-600 is better still at 2.5dB/100' at 900MHz.

 

High quality cable and directional antennas are one of the easiest improvements you can make to a wireless mic system.

 

A passive "sharkfin" type LPDA antenna or a "helical' with 5dB of forward gain, and cable with 3dB less loss is a much better solution than an antenna with a booster, which is basically a place for intermod products to be generated.

 

Mac

[timtheenchanteruk]

cheers, Ill have a closer look/read this weekend to get something, yes it is yeadon cedd, the other couple of places have nothing installed.

 

was looking at the RG58, because I have so much of the stuff already, sure I can find a use for it though, will probably go with the better cable and adaptors...

[john deans]

Some years ago I toook over running sound for a series of conferences, the company I worked (freelance) for had some UHF raio mics with long, 20 to 30 M, RG58 cables which ran down to the front of the venues. They had a lot of problems with odd drop outs, adding the in line boosters improved but did not completely rectfiy the problem. I reduced the cable length to 5M and placed the antennas each side of the operating area on mic stands they worked well for the rest of the time I was involved. I could only assume that the cable used probaly recycled RG58 was causing far more loss that the extra transmitting distance.

[cedd]

yup, if you run the maths, 30m is almost 100ft. Losses at 863Mhz for 100ft are 14.1dB's. Pretty considerable losses.

Useful calculator here; http://www.timesmicrowave.com/cgi-bin/calculate.pl

 

I'm never a huge fan of mast head amplifiers. They have to raise the noise floor as well. If you're going to put them anywhere though, they have to go at the antenna (I know an engineer who had one built into his mic rack, amplifying a 20m run of cable,so he was taking a small signal and making it bigger, rather than taking the larger received signal straight from the aerial, making it bigger and then sending it along the cable). There's a subtle difference and it makes a big change!

I'd much rather use an antenna with gain, and a tight beamwidth. This helps reject interference (decent front to back ratio helps a lot) by not having as much gain off-axis. Your gain is then directed at the stage, rather than if you add gain electronically to an aerial without gain (dipole let's say) in which case your added gain is in all directions. I'd rather trust the lump of metal from a parasitic element in a Yagi antenna than an electronic amplifier.

 

I use the Canford yagi's at front of house, great bits of kit and about 7 dB's of gain I seem to remember. Funnily enough for my VHF kit, I use dipole antennae, but that's because I put my VHF receivers at the stage so you're much closer to the transmitter. I have a feeling the closeness of TX to RX aerial would mean you'd be able to walk out of the beam of a directional antenna. Much better to have an omni (or nearly omni) close in.

 

If you are using an yagi antenna, I tend to tilt mine on their sides by 45'. This is purely as a catch-all as you can't guarantee the polarisation of the TX antenna. Be nice if we could force all the actors to stay upright so the E and H planes are always the same, but it just ain't gonna happen!

[Stan Hope-Streeter]

I'm never a huge fan of mast head amplifiers. They have to raise the noise floor as well. If you're going to put them anywhere though, they have to go at the antenna (I know an engineer who had one built into his mic rack, amplifying a 20m run of cable,so he was taking a small signal and making it bigger, rather than taking the larger received signal straight from the aerial, making it bigger and then sending it along the cable). There's a subtle difference and it makes a big change!

 

This all depends on whether a higher noise floor is going to be more of a problem than an overloaded preamp. Masthead amplifiers tend to overload (and therefore create horrible cross-mods) if the senders are very close to the aerials. So where you should put them may be influenced by how close the aerials are to the nearest performers.

 

And to the OP:

RG58 is OK for short patch leads. The losses in a metre or so of it will be less than those due to the connectors.

[timtheenchanteruk]

not got yagis just yet, just the tat that came with the kit, spent enough on that...

 

will be looking at yagis, or shark fins later on in the year, would prefer something directional, less chance of picking up tat from the pubs on the high street

[mackerr]

Some years ago I toook over running sound for a series of conferences, the company I worked (freelance) for had some UHF raio mics with long, 20 to 30 M, RG58 cables which ran down to the front of the venues. They had a lot of problems with odd drop outs, adding the in line boosters improved but did not completely rectfiy the problem. I reduced the cable length to 5M and placed the antennas each side of the operating area on mic stands they worked well for the rest of the time I was involved. I could only assume that the cable used probaly recycled RG58 was causing far more loss that the extra transmitting distance.

 

This was most likely due to faulty cable or connectors. The loss through cable will almost always be less than the transmission through air loss. Transmission through air follows the inverse square law and can easily have 30dB more loss than through cable. The best solution is have the receivers at the stage with short, high quality, low loss, double shielded, coax to directional antennas.

 

I have a feeling the closeness of TX to RX aerial would mean you'd be able to walk out of the beam of a directional antenna. Much better to have an omni (or nearly omni) close in.

The beamwidth of the LPDA antennas that most of the mic manufacturers sell is in the 80º-90º range to their -6dB point. When you are in close to them you may be outside that -6dB point, but you are much closer to the antenna, so have less inverse square loss. A YAGI might be a tighter beamwidth, but inverse square law is still in effect. If you are in the null of a directional antenna there may be enough loss to cause dropouts even up close, but it will be a pretty specific location.

If you are using an yagi antenna, I tend to tilt mine on their sides by 45'. This is purely as a catch-all as you can't guarantee the polarisation of the TX antenna. Be nice if we could force all the actors to stay upright so the E and H planes are always the same, but it just ain't gonna happen!

By tilting the antenna you are losing 3dB of RF gain for those mics where the antenna is vertical. If you are in a situation where the location of mic packs makes polarization an issue you may be better off with an antenna with circular polarization like the Sennheiser A 5000 CP, or PWS Helical Antenna.

 

If you are using both IEM and mics with helicals, be sure to get right hand polarization for one system and left hand for the other. This will keep the IEM transmitters on stage left from overloading the receivers being fed by the mic antenna on stage right.

 

Mac

[cedd]

If you are in the null of a directional antenna there may be enough loss to cause dropouts even up close, but it will be a pretty specific location.

It's for just that reason that I use Yagi's only at front of house and dipoles at the stage. The beamwidth on my Yagi's from FOH covers the stage pretty nicely, and everything is in the main front lobe safely. Not going to get into another debate about the pro's and con's of having receivers at FOH versus at the stage. In the venue Tim and I are talking about, FOH works pretty well. The venue is only perhaps 20m from stage to FOH and there's very little wing space for a full rack of radio mics, plus mine only have local monitoring, so I can't remotely view RF levels (when I can, I may well change my mind and find a space under te stage out of the way for the rack). I agree wholeheartedly about free space path loss being pretty noticeable, by my calculations (very rough and in my head) you're looking at 60dB's at 100ft.

 

I'd never thought about using polarisation of helicals to keep IEM's and radio mics apart, nice little trick!

 

I'd been looking at the PWS helicals for a while. Can't sniff at 14dB's of gain with that nice tight beamwidth and circular polarisation, but to be frank, I just can't afford one! I'm well aware of the 3dB loss from sticking my yagi's on their side, but it's the best of both worlds. How often do you see a beltpack sat perfectly upright, or with a perfectly straight antenna? It's a trade off I'm willing to make to make the difference between a horizontally positioned mic pack and vertically positioned pack less severe. RF dropout has never been a problem in any of the venues I normally do to date, and signal strength is always good.

 

Stan makes a good point about overloading the input stage of a head amp. I wouldn't ever use one on an antenna with gain unless I'd carefully thought out the levels. Head amps can be a pain, which is why if possible I try and do without (and have managed so far).

[timtheenchanteruk]

TBH, the head amps came with the kit, I'll probably try without first, and see if everything is OK, its oly a 30 second job to whack it in the line if I need to, never needed a head amp with my VHF kit, and I run that through 15M of RG58, never had any signal problem.

 

ahh well, VHF now in the "spare stuff" pile, nice shiny new UHF for me (and yes, its licenced ;) )

[Stan Hope-Streeter]

TBH, the head amps came with the kit, I'll probably try without first, and see if everything is OK, its oly a 30 second job to whack it in the line if I need to, never needed a head amp with my VHF kit, and I run that through 15M of RG58, never had any signal problem.

 

Cable losses are proportional to frequency, so they are a lot less at VHF than UHF. This is why the 213 / 214 cables are specified for UHF systems.

15m of RG58 at 200 MHz would be OK (although 213 would still be better)

The migration of our UHF radio mics from 800 to 600 MHz means that cable losses will be slightly lower, but probably not enough to notice.