radio mic receiving antenna

[timtheenchanteruk]

ok, might be me being dense on a wednesday morning, but flicking through the current CPC catalogue, somthing dawned on my, whilst the old Ch69/70 was above the range of most TV antenna, Ch38 is not, would there be any detriment to using a good quality TV antenna for Ch38 radiomics?

 

Im not buying just yet, but intruiged, as I (whilst not being an expert of radio communications) cannot see why an antenna covering the Ch38 band would not work just as well.

[Sully]

RF wise it should work, the biggest problem is that television antennas are designed to pick up one transmitter some distance away so they're far more directional than what's on your average radio mic receiver (omni or only partly directional) so as long as your artistes only stand in the beam angle and don't move around it should be fine! Quality varies a lot as well, some cheap contract TV aerials aren't very well made to put in bluntly.

 

David.

[timtheenchanteruk]

ahh, never thought of the extreme directionality of a TV antenna, presumably yagi's for radiomics have a slightly wider pattern of pickup.

 

want even thinking of buying a cheap one, they fall apart when you sneeze on them, but the TV yagis did seem alot cheaper than the equivilent for radios.

 

Currently there is no problem round us with other users in 38, the rest of the users are in the de-reg band, or unlicenced 69 users, so easily sorted.

[j_b]

No problem doing this. TV antennae are usually grouped into bands depending on the region they are intended for. Band A antenna for ch 38 use I beleive.

 

If you want it less directional, cut some elements off the front. Simples!

[Jivemaster]

Several antennas for broadcast reception tell you their gain and directionality in their spec.

 

Ask Cedd about cutting director elements off but it sounds a good plan. I did have a single dipole cut from a VHF radio band antenna (88 - 108) by antiferance to suit an old VHF dereg band radio mic (175.00).

 

Directionality could be a good or bad thing according to your location! Too directional and it doesn't cover the width of the stage, simple dipole and it may pick up too much from behind as well.

[KevinE]

How does wave polarisation come into this? TV broadcasts are normally horizontally polarized for main transmitters and vertical for relays...the angle of the TV aerial 'reflecting' this by how its mounted.

[paulears]

Just mount the yagi with elements vertical, works pretty well. The only thing I've always wondered is do receivers that provide 'up the spout' powering for active aerials object to the virtual short circuit of the folded dipole element found in most TV aerial designs?

[Stan Hope-Streeter]

Just mount the yagi with elements vertical, works pretty well. The only thing I've always wondered is do receivers that provide 'up the spout' powering for active aerials object to the virtual short circuit of the folded dipole element found in most TV aerial designs?

 

Yes, it would probably be wise to install a series capacitor somewhere. There should be room in the plastic box where the coax is connected to the folded dipole. I'd guess a 1nF 100V ceramic type would work.

 

I have sometimes used an old (ex TV broadcast relay station) log periodic for radio mics. It's about three and a half feet long and rather heavy, but it covers the whole band from 500 - 900 MHz. At any given frequency, it's roughly equivalent to a 3 element beam.

[cedd]

The only suggestion I'd make is that tv aerials tend to be pretty flimsy in construction. After all, they go up and stay up. I don't know how flightcase friendly a tv type antenna would be.

 

If the specs for the aerials are reasonably technical (doubt it for tv type antennae) then have a look for the beamwidth specification. This is worked out (in basic terms) by taking a power reading on the aerial centreline (transmitting, but pretty much applies the same for reception), then moving off axis until the power drops by a set value (3dB's in my world, but I've heard people quote 6 as well). The angle off centreline you've moved is the beamwidth.

 

To be honest, YAGI's aren't my strong point - we never use them at work cause by definition, most of our aeroplanes tend to be in pretty much all direction, horizontally and vertically. That said, they're basically just a folded dipole with parasitic elements.

Paul's right about the DC - some aerials appear as a DC short, others are open. Plain old dipoles and monopoles in their simplest forms are open circuit DC. That said, as soon as you start adding baluns and the like, DC wise life can get a little complicated. As Tv aerials tend to be a folded dipole (the element is connected to the coax centre at one point, and at the opposite end of the loop, the coax outer, they're a DC short.

 

I could make an educated guess on the effect of removing directors from a YAGI, but no more than that. At a guess, youd see an increased beamwidth but a decreased gain. Don't go moving the reflector (the one behind the main element).

 

To be honest Tim, I use Yagi's in the venue I think you're talking about. They're only 5 element, so not as directional as tv antennae with lots of directors, but the beamwidth of a bought Lintec radio mic YAGI has never been a problem. With the number of reflections at UHF I don't think it would be with tv ones either.

[timtheenchanteruk]

cheers, most of that makes sense, might have a play with a cheepie I have at home.

 

there are some more robust tv types around, a few made for caravanning, so meant for chucking around, Ill have a dig around for the specs, shouldnt be too bad, the better quality ones in the cpc cataloge seem to give the info...

[ramdram]

For those with a more than passing interest on aerials you might have a look here:

 

http://www.hamuniverse.com/yagibasics.html

 

If you can cobble up bits and pieces for spkr brackets, fit out flight cases etc, etc, then you have ample skills to build you own aerial cut for the band/freq your kit works on.

 

The freq/wavelength equations are easy enough, simply google up as nec. Mention is made of impedances and it is fairly simple to grasp the concept.

 

One tip which is probably well known anyway is do not buy cheap with reference to feeder cables (co-ax) and connectors.

 

Just remember an aerial is an aerial...you don't need "special" (aka expensive) for radio mics.

[mksound]

Try using a wideband "grid" type aerial with the reflector removed. They are far less directional and alot more compact for moving about.

We have used many of them on installation jobs to enhance the coverage of the UHF radiomics with great results and great coverage area.

[cedd]

By removing the reflector, do you mean the grid itself? I guess that'd just leave a stacked dipole, which has gain but no directivity (apart from end lobes obviously). A grid antenna with the reflector sounds pretty sensible. 60' ish beamwidth and a good front to back ratio. Being a stacked dipole, I guess they'll also have decent gain, which is handy. Not sure how flightcase friendly they'd be, but for an install, they'd likely do the trick.

 

Of course being a dipole, we'll still have polarisation as an issue. All's well while the transmitter's upright, but get a difficult mic fit (wigs, hats) or a character laying down, and we'll see loss. My solution up until now has been to rotate one of my Yagi's through 45' to at least have half measures. I the let the receiver diversity take care of which aerial it would rather use.

[WooferHound]

If you are using a yagi you can cut it down to 3 or 4 elements and it shouldn't be too narrow for FOH or Booth use, or 2 or 3 elements for onstage use.

I think a 4 element Bowtie antenna would work best but they are big and more suited to permanent installations.

Most lowend receivers do not send DC up the line for amplified antennas, especially the ones that operate on TV frequencies.

[timtheenchanteruk]

Most lowend receivers do not send DC up the line for amplified antennas, especially the ones that operate on TV frequencies.

 

the trantec stuff does, well the antenna distro does, probably not the receiver itself.