radio mic receiving antenna

[cedd]

Yep, my JTS kit does too. Very much low end!

 

Sennheiser do in line amplifiers, but I can't tell if they're powered by the splitter or the receiver. Either way, there's DC on the coax.

 

In fact, I'd say that's most of the big players in the low end of the market listed there, Trantec, Sennheiser, JTS. Shure don't appear to have a cheap product that has removable antennae. Neither do AKG (I'm working on the pricing for a Sennheiser ew100 as being as low as is feasible to go whilst still getting a usable product).

 

In practice of course, lots of tv systems these days have DC on the coax for inline amplifiers and such. Guess there must be a workaround the tv people use. Perhaps the protection is built into the antennae already in the form of a capacitor? Perhaps a device using phantom on the line doesn't pass the DC through to the antenna side of the circuit, and if something isn't using the line, the dc source just sees the short and goes into current limit? Perhaps perhaps perhaps..... tv antennae and coax systems really aren't my thing! If I'm honest, my current radio mic system passes phantom to the antenna. Granted I use the lintec Yagi's, which are a straight dipole and not a folded dipole, so no DC short, but I'd never given it any thought until this discussion, and I'd even go as far as to say most people never do, but for some reason it all just works.

[ramdram]

Further to the slant polarization topic, this might be of interest; scroll down to relevant note:

 

http://www.globalspec.com/Specifications/T...onents/Antennas

 

External FM car aerials are all slant polarized. From lectures on aerial theory decades ago I recall this was to allow for cars going up and down hills etc, in any direction from a tx. The old pre digital portable TVs just used a telescopic aerial at any angle too.

 

The aerials used in radio mics txs are generally any old polarization owing to costume, acting "positions", not to mention being scrunched up in a pouch or pocket.

 

The radio mics (el cheapos) we are using on one of our inhouse productions have a huge range, and work satisfactorily at least ten times the distance between stage and rx position.

[AndyL]

Yep, my JTS kit does too. Very much low end!

 

Sennheiser do in line amplifiers, but I can't tell if they're powered by the splitter or the receiver. Either way, there's DC on the coax.

 

In fact, I'd say that's most of the big players in the low end of the market listed there, Trantec, Sennheiser, JTS. Shure don't appear to have a cheap product that has removable antennae. Neither do AKG (I'm working on the pricing for a Sennheiser ew100 as being as low as is feasible to go whilst still getting a usable product).

 

In practice of course, lots of tv systems these days have DC on the coax for inline amplifiers and such. Guess there must be a workaround the tv people use. Perhaps the protection is built into the antennae already in the form of a capacitor? Perhaps a device using phantom on the line doesn't pass the DC through to the antenna side of the circuit, and if something isn't using the line, the dc source just sees the short and goes into current limit? Perhaps perhaps perhaps..... tv antennae and coax systems really aren't my thing! If I'm honest, my current radio mic system passes phantom to the antenna. Granted I use the lintec Yagi's, which are a straight dipole and not a folded dipole, so no DC short, but I'd never given it any thought until this discussion, and I'd even go as far as to say most people never do, but for some reason it all just works.

 

As with so many things the 'DC power on the antenna cable' question doesn't have a simple answer, even if we only look at one manufacturer. In the case of Sennheiser for example some receivers have switchable DC power on the antenna inputs, e.g. EM 3532, EM 1046, EM 3732 and EM 3731. Some have permanent DC power, e.g. EM 3032, EM 3031. Even when the DC supply is on all those receivers I have mentioned so far have current limiters, so although it doesn't feel like the right thing to do, if you do present them with a DC short on the antenna input then they should cope. But not all receivers have current limiters.

 

Evolution G1, G2 and G3 receivers do not have any DC on the antenna inputs when used stand alone - except for the occasional modified one! - but the antenna splitters ASP 1, ASP 2, ASA 1 and ASA 3000 all do have DC on the antenna inputs, switchable in the case of the ASA 3000. The ASP 1 and ASP 2 do not have any current limiters, but they do have some inductors which burn out if you draw too much current for too long. :)

 

Beware different DC bias voltages. For example Sennheiser use 12V DC these days, but some older Sennheiser receivers used 20V DC. AKG used to use 5V DC - might still, I don't know - which I have seen cause 'distress' (expensive, smoke escaping distress) when people try to use AKG boosters with non-AKG receivers. :(

 

Some Sennheiser antenna boosters do pass DC through. AB 2 & AB 3 do permanently, so they can be cascaded if you must!. AB 1's don't. AB 1036's have a switch inside them so they can if you need them to.

 

And just for the record, putting a DC bias on to an unbalanced circuit in the way we are discussing here is not "Phantom Power". You only get a Phantom circuit with a balanced line. The Phantom circuit is made up by using the common mode of the balanced pair and another conductor, the ground return in the case of phantom power, in such a way that the main balanced audio circuit does not see the "Phantom" signal at all. Using this technique it is possible to carry three balanced audio circuits over only two twisted pairs, a technique once common in telecommunications. All we are doing in powering our active antenna or antenna booster via the coax is separating DC and AC signals by capacitive and inductive coupling and de-coupling. Similarly the DC bias used to power an electret mic when connected to a PC sound card or a radio mic transmitter is not phantom power.

 

I should also mention that TV aerials are designed (allegedly) to have a 75 ohm impedance, which means that they are technically a mismatch to the 50 ohm inputs of most (all?) radio mic receivers. I'm surprised that didn't come up earlier in this thread.

[cedd]

I get what you mean about the term "phantom", but it's still used an awful lot in radio communications (wrongly admittedly). I've certainly heard radio engineers who've been in the business for 45+ years use it.

 

One thing that's really got me rattled is having to have 2 different sets of antennae front of house now for my channel 70 and channel 38 kit. I'm attempting to make use of the 4 I can fit inchannel 70 for less important parts, and so once my channel 38 kit turns up, I'll have to have 2 sets of receiving antennae. Of course, broadband antennae will do the job, but I'd prefer the rejection of a narrowband Yagi.

 

Thanks for the knowledge on Sennheiser's powering arrangements. Definately not straightforward!

[TimmyP1955]

I should also mention that TV aerials are designed (allegedly) to have a 75 ohm impedance, which means that they are technically a mismatch to the 50 ohm inputs of most (all?) radio mic receivers. I'm surprised that didn't come up earlier in this thread.

 

The Shure/Professional Wireless Systems Helical antenna is quite popular over here.

 

 

Some info: "First of all, a 50/75 ohm mismatch is a resultant VSWR of 1.5:1. That translates into 96% forward transmission with only 4% reflected power. Less than 1dB.

 

Even using 50 ohm coax exclusively presents mismatches if its length isn't exactly tuned to the frequency - and only one frequency - it's carrying. Since it's always carrying multiple frequencies, it's always a mismatch, though again by an insignificant amount.

 

Secondly, the antenna you're deploying is varying by as much as 65 ohms (about 37.5 to 100ohms depend on frequency(ies) and near field obstructions); you'd be lucky to see a system VSWR better than 2.0:1 - about 89% forward power.

 

Lastly, the front end of mic receivers, and I don't care what make/model, including the venerable ole' Sennheiser 1046, is not exactly 50 ohms; it's somewhere between 50 and 75.

 

Realty is, your fully "50 ohm" system is perpetually mismatched."  

 

Henry Cohen

Production Radio Rentals

 

 

Also, look for the paper "Random Thoughts from Chicago - Jim Brown: Which Coax for Wireless?"

[AndyL]

The Shure/Professional Wireless Systems Helical antenna is quite popular over here.

 

Mostly used as IEM transmit antennas right? What doesn't seem to be widely appreciated about those particular antennas is that they have quite a narrow bandwidth. If you don't use the correct version for the frequency range of (all) your transmitters it will be more of a hindrance than a help. The correct versions for the lower frequency bands are quite big, with good reason. Those for higher frequency bands, the frequencies you aren't allowed to use in the USA any more, are much smaller. Just 'cos they fit in the flight case easier doesn't make them the right thing for the job. If you are on a 500MHz frequency band you need the right antenna, and if it's a helical it's gonna be a big one 'cos the wavelength at ~500MHz is a lot longer than at >750MHz.

 

As to the importance or otherwise of any mismatch, lets just say that I have fixed more radio mic and IEM systems by replacing the occasional rogue 75 ohm (usually video type) coax that somehow creeps in to peoples systems with an appropriate 50 ohm type than any other single measure. However, that isn't the same thing as a mismatched antenna.

 

Secondly, the antenna you're deploying is varying by as much as 65 ohms (about 37.5 to 100ohms depend on frequency(ies) and near field obstructions); you'd be lucky to see a system VSWR better than 2.0:1 - about 89% forward power.

 

But to which type of antenna was Henry referring to here?[/b]

 

If you have a high VSWR the effect of a directional antenna can be partially or even completely negated by the radiation from / pickup by the feeder cable, so although you might theoretically only be losing ~1dB due to any impdance mismatch the practical side effects can be more serious in an electrically noisy RF environment like a modern live performance stage.

 

As I said, surprised it didn't come up sooner...