Radio mic reception problems (on a boat!)

[Cheeseweasel]

I've recently returned from touring with a travelling theatre company over the Summer, who perform on the deck of a ship (the audience sit on-shore and watch the show).

 

I had no end of reception problems with the radio mics they had on-board - I wonder if anyone could shed any light on this?

 

The rig I was using consisted of:

- 12 AKG receivers (WMS400 if I remember correctly, in any case they were near identical to the current WMS450 units)

- 12 AKG PT400 beltpacks

- 2 Antenna Distribution Units

- 1 PSU

- 2 AKG active paddle antennae (with two more in storage, not being used)

- 2 Decidedly dodgy coax cables (one of which was a 75R video cable!)

 

During the show the cast perform on the deck of the ship and also shore-side. Several of the cast are aerial acrobats and sing whilst flying around all over the place too. The two antennae are positioned on the dock, about 10m apart, covering the forward and aft sections of the ship.

 

I was having problems with dead-spots and generally poor reception at a distance of only 10m or so from the antennae. I'm wondering if this is something to do with the fact that the performance takes place directly above a big steel hull, floating on a large equi-potential plane (creating some kind of Faraday cage around the actors). The problem was further compounded by the need to use rechargeables in the beltpacks (another problem, and major pain, in itself).

 

It's worth mentioning at this point that the company pretty much operate as a pirate ship - all the gear on-board is blagged from distributors and manufacturers - there is no money to buy even so much as a proper coax cable. The radio mic equipment was donated by AKG at some point in the past.

 

So really, I am looking for ways to optimise the system without spending money on it (although selling the two spare antennae has crossed my mind). Getting a proper cable would be an obvious place to start, and I'll work on this over the Winter. I don't know a great deal about the different types of antennae available, but is there a better type for the job, or a better way for me to position them. Also, is there some way for me to utilise all four of the antennae, and how could this be done?

 

Cheers

[cedd]

I'd have a look at the polar diagrams for those antennae. Let us know the model number. Directional antennae do tend to have nulls as you get off of the centreline. With less than favourable conditions, including the losses on the coax (how long were they by the way? What type of cable? RG59 (about 6mm diameter) or RG213 (over a centimetre diameter) - losses at UHF are far from negligible) I can see these lobes starting to make an impact.

 

 

Might sound daft, but try moving the aerials further away. If you can force all the mics into the main forward lobe on the antenna, and not move into the nulls (look at the beawidth spec) you may fair better.

[Cheeseweasel]

Thanks, the antennae were AKG SRA-2B. I've hired them in the past and found them to be very reliable in the small indoor venues in which I've used them.

 

As for the cables, one was a 50ohm RG59, approx. 20m. The other was a 75ohm video cable of some sort, thicker than the antenna cable, but not >1cm, maybe 30m long (there used to be two antenna cables, but one died on me right before a show and I had to grab the next best thing).

 

Whilst I'm sure I was losing a fair bit of signal down the video cable, I was having issues with both antennae. Incidentally, the actors I was mainly having problems with were stood on a big scaffolding structure that ran down the whole starboard side of the ship, which won't have done any favours reception-wise. And even when I put them on-axis to the aerials I had crackles and drop-outs, and moving the antennae further back didn't seem to help. The hand-held that I was using for the soundcheck worked fine from the booth even though I was well behind the antennae, suggesting that maybe the ship itself was part of the problem.

[ramdram]

Strikes me before you do anything else you might want to remake/replace the connectors/cables on your rf feeders. Check connectors in and out of kit. Keep notes of what you do and what works and what does not.

 

Basically start with the easiest to do first. Try substitution and again keep a record. If poss try to set up radio mics system away from ship, as in are they the problem, regardless of where they are sited or only when installed on the ship.

 

If you are able try to borrow a "new" system then try that. It is a process of elimination and will take as long as it takes.

[pisquee]

I worked on ships for 4 years. When we were in a port, or sailing near a coast-line we could have loads of problems with interference from land based radio usage - I guess made worse by different countries allocating radio freqs differently too. Are the channels you are using free of interference and other signals when your transmitters are turned off?

 

How many channels of radio are you using? How are they spread?

 

Are there any other radio systems in use on the ship? (is the ship's radar in operation?)

[StevieR]

Were these performances taking place in the UK or elsewhere? And what frequencies were being used with the mics?

 

There could be the chance of some/all of the frequencies are crossing with local TV channels.

[mackerr]

I was having problems with dead-spots and generally poor reception at a distance of only 10m or so from the antennae.

 

The active AKG paddles have quite a lot of active gain, and about 4dB of passive gain. The first things to check are the cables, and how the antennas are aimed. The "blade" of the antenna should be aimed at the actors. The antenna has about 70º of coverage to the -6dB point. With only 4dB of passive gain (at least that's what the passive version of the paddles have) the coverage pattern should be very much like a cardioid mic, with a strong null directly behind, but fairly gentle rolloff till you get near the null.

 

Your mashup of cables is very likely the problem. They would be the first thing I would replace. With cables 20m-30m long you should be using at least RG213, and preferably something with even lower loss like LMR400. As cables go, these will not be cheap.

 

What are the antenna distribution units? How have you wired the single pair of antennas to the 2 systems? Do the distribution units properly power the antennas?

 

All the suggestions so far have been good.

 

It's hard to troubleshoot over the Internet, but at least you have a few things to try.

 

Mac

[Cheeseweasel]

Strikes me before you do anything else you might want to remake/replace the connectors/cables on your rf feeders. Check connectors in and out of kit. Keep notes of what you do and what works and what does not.

 

Basically start with the easiest to do first. Try substitution and again keep a record. If poss try to set up radio mics system away from ship, as in are they the problem, regardless of where they are sited or only when installed on the ship.

 

If you are able try to borrow a "new" system then try that. It is a process of elimination and will take as long as it takes.

 

The BNCs do need changing. In fact, all the cables need to be thrown in the sea and new ones bought. If I do that job again next year it will be one of the first things I sort out before the tour.

 

I worked through the problem, using different combinations of cable, aerial, transmitter and receiver and came to the conclusion that whilst the cables are rubbish, they're probably not the culprit. An actor positioned 10m away from and on-axis to the aerial shouldn't be dropping out, particularly when my hand-held mic works fine 10m behind the antennae where it's in a null for both aerials.

 

 

I worked on ships for 4 years. When we were in a port, or sailing near a coast-line we could have loads of problems with interference from land based radio usage - I guess made worse by different countries allocating radio freqs differently too. Are the channels you are using free of interference and other signals when your transmitters are turned off?

 

How many channels of radio are you using? How are they spread?

 

Are there any other radio systems in use on the ship? (is the ship's radar in operation?)

 

I did occasionally have problems with picking up local radio. I always made sure that the mics were tuned to clean frequencies before each soundcheck.

 

That AKG system has a number of preset frequency schemes based on country - so e.g. when we were in Italy I was using the Italian frequency set (which I think contains 24 sensibly-chosen preset frequencies to choose from).

 

No other radio systems in use on the ship, though I'm sure there was plenty of radio traffic in most of the places we docked.

 

 

I was having problems with dead-spots and generally poor reception at a distance of only 10m or so from the antennae.

 

The active AKG paddles have quite a lot of active gain, and about 4dB of passive gain. The first things to check are the cables, and how the antennas are aimed. The "blade" of the antenna should be aimed at the actors. The antenna has about 70º of coverage to the -6dB point. With only 4dB of passive gain (at least that's what the passive version of the paddles have) the coverage pattern should be very much like a cardioid mic, with a strong null directly behind, but fairly gentle rolloff till you get near the null.

 

Your mashup of cables is very likely the problem. They would be the first thing I would replace. With cables 20m-30m long you should be using at least RG213, and preferably something with even lower loss like LMR400. As cables go, these will not be cheap.

 

What are the antenna distribution units? How have you wired the single pair of antennas to the 2 systems? Do the distribution units properly power the antennas?

 

All the suggestions so far have been good.

 

It's hard to troubleshoot over the Internet, but at least you have a few things to try.

 

Mac

 

Thanks, I appreciate you looking that up. I'm pretty sure the gear is working ok though, and whilst the cables do need changing they are not the problem.

 

The ADU has an A and B aerial input. One aerial connected to A, one to B. Two other ADUs are daisy-chained to this. From these 3 ADUs, all the receivers are fed with an A and B signal. The aerials are powered properly - the green power LEDs light on both.

 

If we can assume that there is nothing wrong with the equipment (as this is the conclusion I've come to after hours of messing around, changing freqs, moving antennae, substituting parts), does it sound possible that the ship itself is blocking the radio waves? I know you're not supposed to put the aerials near scaffolding and big lumps of metal after all, and a steel-hulled ship is a pretty big piece of metal.

 

If so, an idea I had would be to sell the second set of paddles, buy two omnis and two antennae combiners so I'm using 4 antennae altogether - omni mounted on each mast to cover the deck, and two paddles on-shore to cover the shore action and any dead spots on deck. I'm a bit out of my depth when it comes to antenna stuff, which is why I need help - I understand the theory behind the different types but I don't have experience of anything other than paddle antennae, so any advice would be appreciated.

[mackerr]

If we can assume that there is nothing wrong with the equipment (as this is the conclusion I've come to after hours of messing around, changing freqs, moving antennae, substituting parts), does it sound possible that the ship itself is blocking the radio waves? I know you're not supposed to put the aerials near scaffolding and big lumps of metal after all, and a steel-hulled ship is a pretty big piece of metal.

 

If so, an idea I had would be to sell the second set of paddles, buy two omnis and two antennae combiners so I'm using 4 antennae altogether - omni mounted on each mast to cover the deck, and two paddles on-shore to cover the shore action and any dead spots on deck. I'm a bit out of my depth when it comes to antenna stuff, which is why I need help - I understand the theory behind the different types but I don't have experience of anything other than paddle antennae, so any advice would be appreciated.

 

If you are going to use all 4 antennas with combiners I would stick to the 4 paddles. In order to reduce multipath (which is already being exacerbated by your metal hull) you want to position the combined antennas so the areas they cover are in each other's nulls. You do not want a transmitter picked up by both antennas. You can get good passive combiners from Mini-Circuits

 

With the difference in performance between the HH and lav, there is also the chance that the antenna inside the belt pack is broken. This is not unusual with vigorous use. If you have access to an RF analyzer you can look at real signal strength of the transmitters to help you determine if the transmitter is at fault.

 

Mac

[Paul TC]

If you are going to use all 4 antennas with combiners I would stick to the 4 paddles. In order to reduce multipath (which is already being exacerbated by your metal hull) you want to position the combined antennas so the areas they cover are in each other's nulls. You do not want a transmitter picked up by both antennas. You can get good passive combiners from Mini-Circuits

 

Would not go down the route of two antennas and a simple combiner, unless you can fully seperate the two areas of coverage, e.g. inside and outside a screened box. You may find it makes the situation worse as the signals add destructivly. If it were that simple no-one would sell diversity receivers.

[Cheeseweasel]

If we can assume that there is nothing wrong with the equipment (as this is the conclusion I've come to after hours of messing around, changing freqs, moving antennae, substituting parts), does it sound possible that the ship itself is blocking the radio waves? I know you're not supposed to put the aerials near scaffolding and big lumps of metal after all, and a steel-hulled ship is a pretty big piece of metal.

 

If so, an idea I had would be to sell the second set of paddles, buy two omnis and two antennae combiners so I'm using 4 antennae altogether - omni mounted on each mast to cover the deck, and two paddles on-shore to cover the shore action and any dead spots on deck. I'm a bit out of my depth when it comes to antenna stuff, which is why I need help - I understand the theory behind the different types but I don't have experience of anything other than paddle antennae, so any advice would be appreciated.

 

If you are going to use all 4 antennas with combiners I would stick to the 4 paddles. In order to reduce multipath (which is already being exacerbated by your metal hull) you want to position the combined antennas so the areas they cover are in each other's nulls. You do not want a transmitter picked up by both antennas. You can get good passive combiners from Mini-Circuits

 

With the difference in performance between the HH and lav, there is also the chance that the antenna inside the belt pack is broken. This is not unusual with vigorous use. If you have access to an RF analyzer you can look at real signal strength of the transmitters to help you determine if the transmitter is at fault.

 

Mac

 

Cheers, I'll get hold of a load of new cable, a couple of combiners and see how it goes then.

[ramdram]

Just a thought...have you asked the RO aboard ship? I'll presume (s)he will have some idea, not only on the theory of (marine) wireless but also should have some idea about fixing the ship's systems whilst at sea.

 

Could you cadge some cable and fittings off the ship's inventory? I can't imagine the owners are that cheap/poor not to be able to lay in some spares???

 

If not then do you know if the lifeboats "work"...er...can you swim? ** laughs out loud **

[Cheeseweasel]

Just a thought...have you asked the RO aboard ship? I'll presume (s)he will have some idea, not only on the theory of (marine) wireless but also should have some idea about fixing the ship's systems whilst at sea.

 

Could you cadge some cable and fittings off the ship's inventory? I can't imagine the owners are that cheap/poor not to be able to lay in some spares???

 

If not then do you know if the lifeboats "work"...er...can you swim? ** laughs out loud **

 

Haha, it's just not that kind of ship. It's more of a floating hippie commune. The ship's crew consists of 8 cast members, 2 cooks, engineer, captain and his wife, sound, lx and video techs, riggers and admin guy. I'm probably the most qualified person on-board to deal with radio stuff tbh and that's saying something.

 

And the owners survive off the donations we were given at shows. After running costs were deducted, everyone on-board, including the owners was paid 20 euros a week! If I'm to get new cable I'll probably have to blag it for free from some kindly company! Btw if anyone has any 50ohm low-loss cable gathering dust in their garage, give me a shout!

[Gareth Owen]

Okay, here is a left-field suggestion : I once had a problem with RF on a large boat. It was an install so we had a tonne of time to work out the problem. The Ariels were hex-clamped to a metal rail on the upper deck. It turned out that the metal transducer of the echo-sounder wasn't properly isolated form the metal hull of the boat and the high frequency bleed through the hull was disrupting the much lower frequency radio mic channels. It took us days to work out the problem. Ironically, once we did, and got it fixed, the echo sounder worked much better too!

[ramdram]

Ref # 13

 

Crikey! Just to misquote Terry Pratchett, hope you are only "Going Coastal?"