Question regarding Log-Periodic Dipole Array/ Sharkfin

[Envoy]

I have seen production companies using only (1) LPDA for their wireless setup.

 

I thought a pair was needed? It appeared they were trying to get one paddle -to do the work of two.

 

Is this possible? Any answers will be greatly appreciated.

[Bobbsy]

A pair is needed for proper diversity reception but, if the signal is really good/strong it's possible (though a bit silly) to get away with one.

 

Are you sure they were just using a single antenna though? Diversity works best if the antennae are well separated so the other one might have been the other side of the room.

[peterskine]

I have seen production companies using only (1) LPDA for their wireless setup.

 

Please post more information.

[cedd]

You may find that they were using a different type of antenna, maybe a far smaller, less obvious one, on the other side of the diversity receiver. This can sometimes be quite a wise thing to do. For instance, I might use one log periodic across the stage to cover the majority of the action, but if there's occasion for a mic to end out in the audience, I might stick a dipole on the other side of the receiver to make sure I cover the audience well. LP's have quite a narrow beam width so an omni antenna is useful to counter this. This antenna may even be the supplied little one that came with the receiver.

Both sides of the receiver being connected to an aerial isn't essential for them to work. The receiver in simple terms just picks the aerial input with the best signal. I've never had to do it, but I'd maybe be tempted to stick a 50 ohm load across the unused input to stop it acting like an antenna. If the aerial was a decent length of cable away from the receiver and the mic user walked right up to the receiver rack, there's a chance that the RF would be of a sufficient level to make the receiver switch to the unused port instead of the distant antenna. Results from this side of the receiver without an aerial could be quite unpredictable, so it's probably best to force the receiver to use the proper antenna by loading the unused connector.

[peterskine]

I have seen production companies using only (1) LPDA for their wireless setup.

 

Sorry, what I meant to ask was, Please Post more information about the setup you saw where it seamed to have only one antenna with only a single coax connected. Did you take any pictures? did you see that only one antenna was connected to the RX?

 

I have seen dual antennas which are built onto one LPDA with two coax connected, the second antenna is omni sticking out the side of the fin. I do not endorse doing antennas this way but it does exist.

[paulears]

and totally pointless - the point of diversity reception is to try to avoid blackspots - separation is needed. Using two aerials of different directional properties makes sense sometimes, but putting them in the same place seems rather a waste of time.

[ramdram]

Putting both aerials in almost the same spot is not space diversity. Space means "space" literally as in physical distance between aerials. You might get, also, refinements such as different polarization (think of the aerials plugged into the back of the receiver at 90 degrees to each other).

 

When I was "working" (Monitoring Service) we used space diversity...quite a few separate aerials to select...and frequency diversity...even more frequencies depending on conditions. Our space diversity aerials were hundreds of metres apart and the frequencies for some Russian bx numbered in the dozens. That is true diversity.

 

As mentioned before in many other threads you might care to have a squint at the Ham Radio sites. They are some of the best sources of anything technical to do with "radio" without going into pages of math.

 

(You will find any number of designs, for aerials, yes, even log periodic, you could cobble up with bits of wire and coax, which will work just as well as the kit off the shelf at a mere fraction of the price. Might be worth a punt?)

[graemeftv]

Having a pair of non identical ant connected to a radio mic receiver is not always wise:

 

1: having one ant over stage area and one in audience means that when the rf is in a black spot on the stage or the rf is relecting from a wall etc at a certian point the reciever might not want to switch to ant B located in the audience as the rf signal my be worse due to the extendend distance of the ant from the rf source.

 

2: 2 x ant over stage area, several meters apart from each other. 1 high gain active paddle with 10db internal amp on 5 m cable, other passive dipole on 10 meter cable. The receiver will always tend to stay with the stronger rf (active paddle) therefore both ant need to be identical with same cable lenght ideally. Also hard to explain but its not always about how far the mic is from the ant. its about one ant receiving reflections or wave intermodulation from other mic at that moment in time. clearly its works fine 99% with mixed ant if the whole setup is in a small space with only a few radio mics.

 

3: same ant setup as case 2

the worst case is with a diverity radio mic receiver, but not 'true diversity'. so say 'rf A' is poor at one black spot on stage, reciever doesnt know that the signal on 'rf B' is poor because receiver doent contain 2 complete rf circuits so cannot compare signals. so reciever switches to rf B becuase rf A is weak. Receiver then switches back to Rf A which it doenst know if its better or not than Rf B - and so the switching can continue fairly rapidly until the rf source moves. Clearly this can also happen with 2 identical ant setups, But its much worse when one ant is always weak compared to others.

 

a fair number of the imported radio mics tend to not to be true diversity. some examples: trantec system II of the late 1990's was true diversity, s2000 was. but later s3.2, s3.5 s3500, s4.5 and s4000 were not true diversity.

[peterskine]

It is, as I stated before, not a good idea to have a dual antenna. Probably only slightly better than a single antenna and definitely not as good as two antennas spaced at least a wavelength apart.

 

Here is the antenna Log-Periodic Dipole array

 

This same company does make some interesting antennas to bury in the stage under the performers, though.

I have been in congested RF environments where the only frequency I could add was on or

near a weak DTV station and it worked as long as the TX was within a few feet of the RX.

This kind of antenna would help.

[paulears]

Ah - I see. Not an omni really, but a horizontally polarised dipole. - so directional used on it's side. So in effect it's a sensible idea with horizontal and vertical components helping to reduce nulls. Log periodics have a bit of forward gain, 5dB being quoted, but the two horizontal components of the dipole have 1.8dB gain over a dipole, which suggests they're not 1/4 waves but some kind of compound antenna? If you don't have the option of splitting them up - and I'd suggest a wavelength isn't sufficient, they'd be quite useful. My own experience is that rarely is signal strength that different between two aerials - watching the receiver swap inputs show that frequently the thing doesn't switch until the signal takes a sudden dive into the noise. That's what I don't like receivers working out front like some people do - once the signal goes noisy, the chances the other will be noisy too. A long cable to an aerial up high, and the other even being the one attached to thee receiver seems to work fine for me - far enough apart that the chances of the nulls for both antennae being at the same point are slim. In addition as the transmitter gets further from one it gets closer to the other. Decent feeder is ideal, but I once did a TV job where instead of a diversity receiver, we tried a combiner. The reason was that the job needed to cover a presenter walking towards the camera on a towpath next to a canal - talking to camera. We put a dipole in a bush around 200m away, and ran a very long length of Westlake 103 to the combiner, the other input to the combiner went to another dipole local to us. It worked perfectly - 200m of cable is an awful lot of loss - at least 20dB, but it worked really well - the signal from the presenters mic starting to reduce as he walked towards the camera, but very soon, the level to the local dipole started to go up and we didn't have a dip in the noise. I guess my point is that signal strength rarely reduces gradually - but takes a big dive as you get into a null path. Cable loss is often made up for by a better path to the aerial.

 

I wonder if a simple pair of dipoles at 90 degrees to each other would work well? I might even give it a shot.

[peterskine]

No matter what the distance, coax with any loss has less loss than free air. I often duplicate RX antennas on the same line to customize or extend the coverage area. On a 300ft LMR400 coax a 10 db gain active paddle does quite well at the RX

[Bobbsy]

In TV it was pretty common to use Sony radio mics--and their antenna combiner/splitter unit could take up to 4 antennaes in and give two lots of five outputs each (which needed termination if you weren't using them).

 

I recall on BBC kids show we did where they wanted to use out studio but also the green room, reception and corridors linking all of the above. We had to work out antenna locations to cover the who route--no diversity, just four carefully positioned antennas (and lots of low loss coax) with the receivers switching among them as required. It actually worked.

[paulears]

As I've said many times - I routinely use passive splitters - ex-offshore oil/gas industry kit I've had for years - and cannot notice any difference in performance between racks fitted with manufacturers DAs and the passive ones. Very rarely do we suffer from free space path loss as a problem - it's virtually complete loss of signal as people walk into nulls or the path gets broken.