Review: BNC connections, Slim Jims and ribbon J-Pole portable and emergency aerials

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BNC connections, Slim Jims and ribbon J-Pole portable and emergency aerials and responds to some reader feedback from South America.

Reviewed by:  Keith Rawlings G4MIU

keith.g4miu@gmail.com

 

Get Connected and Get Outdoors

 

Once again, Hello and welcome to this month’s Aerials Now. I hadn't planned to start anything on the subject of connectors just yet. However, after a slight faux pas on my part recently, and following some feedback from reader Steve, regarding the Mini Whip from the March column, I thought now might be a good time to do so.

 

I will start with the 'BNC'-type of connector. ‘BNC’, I believe, stands for Bayonet Neill–Concelman.

 

A few months ago, I decided to have a ‘shack tidy-up’, to make things neater (!) My receivers were all temporarily put away while this was taking place. Well, things have taken longer than planned, and one or two receivers have, therefore, ‘crept back’ into use.

 

One of them is my trusty old AOR AR2001, which I leave monitoring on a couple of channels. I put it back into service with a male BNC-BNC extension lead that runs from the aerial input on the set over to the connection for my discone, which uses a BNC plug. A quick check on the nearby Stansted ATIS channel proved all was working OK, so I entered my frequencies and left it running. After a few days, I realised I hadn't heard very much, so I tuned back to Stansted and received nothing! I made some checks; when I moved the set, it burst back into life all of a sudden. The aerial connection was intermittent.

 

A quick look at the lead and it all came back to me: I recognised this as a 75Ω lead from an old military signal generator I have, and which should not have been used here.

 

Mating Ohms

The issue is that 50Ω and 75Ω BNC connectors are slightly different; while it is possible to make them ‘mate’, I try to avoid this because the pin of a 75Ω plug is slightly thinner than the one on its 50Ω counterpart. When it connects to a 75Ω female socket, there is obviously no problem. But if it is used with a 50Ω socket, you may well get the issue I had. The (larger) 50Ω pins have a bigger aperture on the socket. Therefore, the smaller 75Ω pin connector can make intermittent contact, especially if the socket is a bit worn. This is probably the case with my AOR.

 

On the other hand, using a larger-pinned 50Ω plug with a 75Ω socket may damage the female contacts.

 

Most good-quality BNC connectors will have either ‘50Ω’ or ‘75Ω’ imprinted on them. The 75Ω connectors are commonly being used in applications such as CCTV, networking and some older commercial radio equipment.

 

Loving the BNC

I admit to liking BNC connectors: They provide a constant impedance over their operating range (up to 4GHz), and they make a secure connection, by virtue of their bayonet fitting. They are easy to fit, release with just a twist and (getting to reader Steve's correspondence), they are (in general) waterproof.

 

However, for continued exposure, they will need some form of protection, either in the form of a 'rubber- boot' or a coating of liquid rubber or self-amalgamating tape.

 

Bear in mind, though, that the latter two options can make them difficult to disconnect, as the coating may have to be removed and then sealed again for further use.

 

BNC connectors are simple enough to put together. Note, however, that the ‘crimp’ types will need a suitable crimping tool to assemble them correctly. They may not be as water-resistant as the compression-type connectors (Fig. 1).

 

 

Pictures: Keith Rawlings

Fig.1: Crimping Tool with Crimp BNC connectors.

There are also different connectors available to match the different sizes of coaxial cable.

 

These links lead to some general details for assembling the compression-type BNC connectors:

 

www.solidsealing.com/technical/documents/NS10551.pdf

https://www.youtube.com/watch?v=DksUM656s-M

And for the crimp type, you can find more information here:

https://www.youtube.com/watch?v=ktQVwfo-s9w

https://www.youtube.com/watch?v=NVIIKap9jYs

 

I used to prefer the compression-type, but I got reasonably good at fitting crimp-style TNCs. These are similar to BNCs but are ‘threaded’. I worked with those when re-fitting new radios and aerials on some 350 British Gas vans, around 19 years ago!

 

The image in Fig. 2 shows one arrangement I use in my garden, for quickly connecting to my experimental setup for HF aerials. It consists of a short run of RG58 cable to the shack, with a compression-type BNC plug on each end. In the photo, the compression BNC connector on the feeder to the shack can be seen at the bottom, and this is presently connecting to a female 'flying' BNC crimp connector, running to a mini Whip. To keep the weather off, the connectors are pushed up into an old milk carton, just visible. This method of weather-proofing has been very effective over a number of years.

 

 

Fig. 2: BNC Male Compression Plug fitted to a Crimp BNC Female to the top.

BNCs have been amongst my favourite RF connectors for many a year.

 

By contrast, my most disliked variety has to be the old PL259-style and SO239-type connectors, although modern alternatives are an improvement.

 

More on connectors next month.

 

Aerials for the Job (ctd.)

After a break last month, for the CCW active indoor loop aerial evaluation (RadioUser, May 2019: 62-64), I can now get back to the subject of choosing aerials. As the better weather is, hopefully, fast approaching, now may be a good time to think about aerial projects.

 

For me, this is the time to start thinking about taking radios out portable again. I find it quite relaxing to find a quiet spot in the countryside, on the outskirts of the village, put up an aerial and operate, either on the amateur bands or to engage in a bit of 'scanning'.

 

Considering the VHF and UHF bands, my ‘go-to’ aerials for portable (and emergency) use are simple J-Poles, constructed from ribbon feeder. These give me the ability to transmit on 2m and 70cms. They can also provide decent reception quite some way either side of those bands, including airband and PMR.

 

To get an improvement in performance for the reception, I made a fold-away aerial. I then bought a Moonraker Australia BRX500MP, described as a 'wideband-receive only man-portable tactical or covert surveillance aerial'. This is now my main receive aerial for portable or emergency use. It is an excellent choice to consider – if you can find one (Fig. 3).

 

However, the BRX500MP is not suitable for transmitting, and my J-poles still feature strongly for this.

So, what could be a suitable aerial for portable or emergency use on VHF/UHF?

 

Portability Matters

Well, there are a lot of contenders out there, and people will have their own preferences, but (since it works well, is easy to make and easy to deploy), I still like the ribbon J-Pole. My original designs used cheap, translucent, 300Ω ribbon feeder, housed within 22mm overflow pipe, and a short tail of RG58 coaxial cable with a flying BNC female connector at the base.

 

Since then, I have used the sturdier, slotted, type of feeder, for 300Ω and 450Ω, with a similar arrangement for the feeder but without a casing. This makes it possible to roll up the aerial when not in use and even stow it away in a pocket when on the move.

 

A design for the 2m amateur band using the 450Ω feeder, which also works well enough on the VHF and UHF PMR bands, uses a 1493mm length for the main element and a 497mm length for the stub. The spacing of the elements, as constrained by the 450Ω feeder, seems to work well. The coaxial cable is attached approximately 50mm up from the bottom of the main element (Fig. 4).

 

Fig. 4: Basic diagram of a J-Pole. See M0UKD’s website for design details: https://m0ukd.com/calculators/slim-jim-and-j-pole-calculator

 

To get the J Pole exactly on resonance may take a bit of experimentation, by means of moving the feed point up or down, and/or trimming the 1/4 'stub'. The dimensions given above were found using modelling software with the dimensions entered in λ (wavelength), which was then converted to millimetres.

 

They agree closely with M0UKD's online calculator and construction details which may be found here:

https://m0ukd.com/calculators/slim-jim-and-j-pole-calculator

 

This calculator can also work out the dimensions of the similar Slim Jim designed by Fred Judd G2BCX (SK). I met Fred many years ago when he gave a talk for the Cheshunt and District Radio Club on the Slim Jim and also his ZL Special. This was the first time I had a seen demonstrated a fluorescent tube, energised not by a light switch but by RF from a radio! Shortly afterwards I made my first Slim Jim, using 300Ω ribbon feeder; I did this with some difficulty, I might add, but I got there in the end.

 

Overall, I prefer the J-Pole when using ribbon feeder, as it seems easier to adjust than the Slim Jim. I used a ribbon feeder for convenience. However, any suitable material may be put to use. I have successfully used 1/8" diameter brass rod, cut and soldered, for use with my Radarbox on ADS-B.

 

A rolled-up J-Pole may be deployed almost anywhere, hung from a tree, or on a curtain rail in a hotel room; even by a window at work, if you can get away with it!

 

RG58 is a convenient choice for feeder, but longer runs may be bulky. If this is an issue and you only need a very short run of feeder, try RG174. It will be lossy, compared to RG58, but it may be more convenient. RG316 may be a (more expensive) substitute with better performance.

 

If you choose to place a Slim Jim or J-pole in a housing, be prepared for the aerial to be de-tuned.

 

Although not ideal, I have used 22mm PVC overflow pipe to cover a J-Pole. Glass fibre would be better.

 

The computed plot of my 145MHz J-Pole can be seen in Fig. 5.

 

 

Fig. 5: 145MHz J Pole Far-Field Plot and SWR Plot.

On the left, at its design frequency, can be seen the low SWR and low radiation angle at 145MHz.

 

On the right, the SWR plot at 432MHz can still be seen to be low. However – if you look at the Far Field plot above – it can be observed that most of the signal is going upwards.

 

For comparison (also shown) you can see the performance at 313.55MHz. Here, the SWR is high, but the angle of radiation has both low and high spots.

 

The radiation pattern will change, depending on frequency. Best performance is at the design frequency. I have found the ribbon J-Pole to be a good general-purpose VHF/UHF receiving aerial, which is easy to make.

 

Feel free to email me and tell me about your favourite portable or emergency aerial.

 

Feedback

It is always nice to hear from readers. Paulo PU2RDX, from Bauru City, near Sao Paulo in the south-eastern part of Brazil, introduced himself to me (see RadioUser, Feedback, April 2019: 68). Paulo is a keen radio amateur and has been a  listener and DXer since he was a kid. He asks whether I can, “unlock the secrets  behind isolation transformers, galvanic isolators and common mode chokes to help fight interference.”

 

Paulo, I can answer this with a ‘yes’, as these are amongst a large range of subjects I wish to cover in future columns. In the meantime, can I refer you to my BALUN article in the September 2016 issue of RU, which included the design of an isolated LF/HF BALUN (RadioUser, September 2016: 58-63).

 

As always, so that everyone may benefit, I will reply to correspondence via this column.

 

Until next month, Good Listening.

 

This article was featured in the June 2019 issue of Radio User.