Aerials for Radio Mics

[Lamplighter]

I was looking to improve my receiver aerial arrangements on channels 38 and 70 when I spotted this which seemed to have some possibilities. I bought one to experiment with and was pleased with the result.

 

The aerial element assembly is plugged into the stand body by TV style connector. I made a few small changes and now have a very cheap "paddle" The impedance match to 50ohms is pretty good at channel 38 (14dB return loss) and excellent at channnel 70 (+22dB return loss ) and there is a gain of at least 4dB at channel 38 and 6dB at channel 70 compared with my Trantec supplied whips. I haven't access to a proper aerial test range so the gain figures are a somewhat arbitary. The performance is comparable to a comercial paddle, though not as rugged.

 

Modifications.

 

Remove the end cap of the boom and undo the 6 scews holding the elements, noting which are proper screws and which are self tappers! There is some adhesive holding both the end cap and also the aerial blades but the bond can be broken by hand with a good waggle. Once the blades are removed open the plastic cover to gain access inside the boom arm. Fit a 10nF ( CPC part no CA05510 ) isolating capacitor in series with the inner conductor where it joins the coppered contact at the front end of the aerial. This resolves any DC power issue with the receiver or distribution amplifier. I also fitted a 2k2 resistor ( CPC part no RE04074 ) across the input socket to allow a feeder continuity test after installation. I also fitted a BNC adaptor (CPC part no PSG01745 ) to the input socket. Before fitting the adaptor, gently squeeze the inner and outer of the coax socket to ensure a tight fit. Apply a small amount of superglue or similar to the barrel of the adaptor before fully seating it. Clean the copper contacts and the mating surfaces of the aerial blades with wire wool and then reassemble. Wind 2 turns of 19mm wide electrical tape tightly round the front end of the boom. This ensures that the aerial elements maintain good contact as well as holding the nose cap firmly in place.

 

Mounting

A 40mm long section of 20mm square steel tube with a 4mm hole drilled in the centre of one diagonal and an M4 nut welded over the hole makes a suitable clamp for the rear of the aerial. This section can be readily welded or bolted to the desired mounting. I glued a 40mm piece of 12mm aluminium angle to the aerial boom, with a flat filed into the centre of the angle where the clamping screw touches, this gives a very positive fixing.

 

Typically, when I had completed a set of 4 (2 x Chan 38 plus 2 x Chan 70 ) another CPC offer pops through the letterbox. With offer code 08 the aerials are now £7.85 +VAT. I suspect that they are clearing out their stock as the newer ( more expensive) version incorporating a 4G filter has appeared. There is an irony in that somewhere!

 

Does anyone have a use for 12 telscopic rod aerials? I also had a play with the amplifier that is built into the stand but found it is a receipe for death by intermodulation when used with radiomics.

 

Brian

 

 

The link seems problematic, CPC code AP02369 is the item in question.

 

Moderation: link fixed

 

PICTURE Close Up

[Stuart91]

Your original link was missing a colon, here's the corrected link.

 

http://cpc.farnell.com/antiference/ss100-s/aerial-indoor-high-gain/dp/AP02369?Ntt=ap02369

[paulears]

The real issue with this is that by design the log periodic aerial is a wideband device, and handy for portable use in strong TV signal areas because it covered the entire UHF TV band. The biggest problem isn't so much now, but when 69 gets full of mobile signals, there's no filtering to attenuate these, so you will be presenting your receivers with all sorts of non-radio mic, strong local signals. Much depends on how good your receivers are at filtering this out, but the idea is worth a go, at the price - but there again, so would many others like this kind of thing. It could also be easier to mount?

[Lamplighter]

The problem with lack of filtering is little different from the manufacturer supplied 1/2 wave aerials, With a directional aerial you can take advantage of the pattern to increase the wanted signal and reduce the unwanted providing that you are able to select the optimum mounting position. I don't see any problems with channel 38 in that my aerial distribution is well filtered and I can readily add a brickwall bandpass for channel 70. I chose the particular aerial as it is easy to transport and mount 2 pairs, the Paragon aerial would give very similar results if they were considered mechanically more suitable. On of the intersting discoveries was finding that the Antiference aerial used 50 ohm cable internally and it is far too long to be a matching section.

Brian

[soundspider]

Paul, I was just looking at the specs for the aerial you linked to, and it says it's frequency range is 470-862KHz. Does this mean it is completely useless for Channel 70 or will the signal simply tail off the further you get from the centre frequency (666KHz, spookily enough, if my maths is/are correct)?

 

Alan

[paulears]

They're 21-69 and with Log Periodics the fall off outside that range isn't that sharp, because unlike resonant aerial designs, they don't have much gain really. The original the OP used is made by antiference, a pretty established aerial maker, who avoid giving any spec on their product at all, but based on the size, I suspect they have a fairly gentle directional pattern. The other seems to have a proper front/back ratio.