Thoughts on this please.....

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SOTS 87 7
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Thoughts on this please.....

Post by SOTS 87 7 » Tue Sep 03, 2019 10:55 am

I came across this on eBay, seems quite expensive but well made.
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Albert H
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Re: Thoughts on this please.....

Post by Albert H » Tue Sep 03, 2019 11:58 am

The length of the phasing loop doesn't look quite right - perhaps it's the perspective.

You do know that you can make a J-pole yourself for <£10 with materials from B&Q or most other DIY stores? I used to make them out of copper plumbing pipe, with a cut-up thick plastic chopping board used for the spacers between elements.

I had a test set-up in my garden that let me put the aerial under test about 3m up in the air and well away from the house, shed and any trees. I had a clean little 1 Watt transmitter that was matched accurately to 50Ω and had a built-in SWR bridge. The test transmitter could be tuned anywhere in the band and allowed very accurate adjustment of the aerial before it went to site!

Incidentally - look up the "copper cactus" aerial. It's really easy to make and has useful gain.
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Re: Thoughts on this please.....

Post by XXL » Tue Sep 03, 2019 5:11 pm

I know exactly who made that.

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Re: Thoughts on this please.....

Post by SOTS 87 7 » Wed Sep 04, 2019 9:16 pm

Exactly Albert, The rather odd length of coax threw me a bit. yes, I've made one or two J poles before for 2 meters etc, and had some fantastic results. Can't say I've ever known about a "coax balun" ever fitted to one.
Go on Mr.XXL Who makes them? I will say they look nicely constructed.

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Re: Thoughts on this please.....

Post by RF-Head » Wed Sep 04, 2019 11:13 pm

please NO names

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Re: Thoughts on this please.....

Post by SOTS 87 7 » Tue Sep 10, 2019 5:30 pm

Absolutely not.

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Re: Thoughts on this please.....

Post by XXL » Wed Sep 11, 2019 11:21 am

I won’t say who made them but they are fucking awesome. A regular pirate unmatched dipole compared to this is no comparison. I’d rather buy that than a more powerful rig tbh.

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Re: Thoughts on this please.....

Post by thewisepranker » Wed Sep 11, 2019 12:38 pm

An unmatched dipole is lousy when compared to pretty much anything.

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Re: Thoughts on this please.....

Post by Albert H » Fri Sep 13, 2019 10:13 am

The cheapest power amplifier is a well-matched antenna (and feed system). Also, don't waste your expensive power in inappropriate (PL259 / SO239) connectors. You can lose a significant amount of power just heating up those crappy connectors!

It's even better if you can use an antenna with gain - a dipole (when correctly matched) radiates a Watt for an input Watt. A dipole is convenient and cheap, but can easily be bettered at little cost. A stacked dipole shapes your coverage and gives gain very cheaply (if you get the dimensions and matching right), and a quad stack works even better. Professional installations always use aerials with gain (and shaped coverage), or if they're British and afflicted with the insane desire to transmit "circular" polarisation, they use lossy aerials. It should be noted that "circular" polarisation doesn't actually do anything to mitigate multipath distortion (it actually worsens it) and most listeners will be using vertical aerials!

I always used vertical aerials up blocks (except for link receive!) - the ⅜, ⅝ and ⅞-wave aerials all provide very useful gain and don't look obvious from the ground. The J-pole generally matches better than a dipole, and is (effectively) an end-fed half-wave. It shouldn't really give gain over a dipole, but because it's easy to match correctly, doesn't require a balun and puts the power where you want it, it's a great choice for clandestine broadcasting and always works better than a dipole.

Back in the 80s, a certain well-known engineer was astonished to see how little power we could get useful coverage with. I put a little rig (about 80 Watts as I recall) up a block on Sydenham Hill and got better coverage than his 400 Watts into a stacked dipole on Church Road Crystal Palace. The field strength difference over most of Central London was the difference between us being able to run good-sounding stereo and his signal being too noisy to do that except close to his site. Our aerial was too tall (just) - even when broken down - to fit in the lift, so we pulled it up the side of the building with a rope! Gotts thought that the signal was coming from Peckham because it was so strong there!
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Re: Thoughts on this please.....

Post by XXL » Sun Sep 15, 2019 1:19 am

What connectors would you recommend Albert ? They all seem pretty lousy tbh.

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Re: Thoughts on this please.....

Post by Albert H » Tue Sep 17, 2019 6:36 pm

XXL wrote:
Sun Sep 15, 2019 1:19 am
What connectors would you recommend Albert ? They all seem pretty lousy tbh.
Low power: 50Ω BNC connectors work well. They also fit RG58 coax easily. These remain pretty low loss up to about 20 Watts.

Link TX / RX Band IV / V: TV "Belling Lee" connectors - they're quite low loss, and for the tiny RF power that I used to transmit up my links into big TV Yagis, they work fine. They're also 75Ω which is convenient for TV downlead and connection to ordinary, off-the-shelf TV aerials. UHF is really brilliant for linking - you can go for miles on milliwatts, the aerials and feeders can be bought for next-to-nothing from your local DIY superstore, and nobody notices a TV aerial on a roof - even if it's pointing the wrong way!!

Higher power: N-type. These are not much more expensive than the PL259 / SO239, and tend not to melt at a few hundred Watts! They also match the coax so well that the discontinuity is difficult to see on a TDR. The best ones use a derivative of Teflon for the insulators and don't get hot in use. It's also easy to wire these on to the end of WF103 (or whatever flavour of fatter coax you're using).

When you get up into the many kilowatts, connectors become a bit specialised, and are generally supplied as part of the system by the manufacturers.
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Re: Thoughts on this please.....

Post by thewisepranker » Tue Sep 17, 2019 8:39 pm

Before I start, I completely agree that the SO-239 and PL-259 are terrible and not fit for purpose.
Albert H wrote:
Tue Sep 17, 2019 6:36 pm
Link TX / RX Band IV / V: TV "Belling Lee" connectors - they're quite low loss, and for the tiny RF power that I used to transmit up my links into big TV Yagis, they work fine. They're also 75Ω which is convenient for TV downlead and connection to ordinary, off-the-shelf TV aerials. UHF is really brilliant for linking - you can go for miles on milliwatts, the aerials and feeders can be bought for next-to-nothing from your local DIY superstore, and nobody notices a TV aerial on a roof - even if it's pointing the wrong way!!
"Belling Lee" or "IEC 61169-2" connectors are worse at 75 Ohms than PL-259s are at 50 Ohms. On the face of it, you might think that they are pretty cock on 75 Ohms when you look at the dimensions:
Male - Copy.JPG
We take dimension "A" (the male pin) and dimension "B" (the outer conductor) and put it into an impedance calculator. Assuming εr = 1.001 for air, this works out to be 77 Ω. Seems they got it as good as spot on when they were designing the connector, right?

Well no, because this is not how the connector sits in the real world. It sits like this, where the dark part is the female connector:
connected - Copy.jpg
The impedance of the interface is between the outside diameter of the female pin and the inside diameter of the outer conductor of the male connector. This isn't dimensioned on the free versions of the IEC 61169-2 standard that I can find on the Internet:
both dimensioned - Copy.JPG
We can get a rough idea, though. We know dimension "A", which is the outside diameter of the male pin. The hole in the female pin has got to be larger than Ø2.35 mm to allow the male pin to fit. We can assume the hole to be around Ø2.5-2.6 mm. The female pin is formed into a taper after machining to reduce the front section in diameter to just under Ø2.3 mm in order to ensure good contact.
Allowing for a bit of wall thickness we can assume that the outside of the female pin to be somewhere around Ø3.5 mm.

It works out at 53.2 Ω.

You can check this by looking at return loss specs on high quality, commercially available connectors that actually have spec sheets. Here's an example:
https://www.hirschmann-multimedia.com/m ... 0v17.2.pdf

Return loss spec. is on the second page:
>20 dB at 40 MHz, -1.5 dB/octave
This is 14 dB at 640 MHz (1.5 SWR) and obviously gets worse as you stray further into Band V.

I think F connectors are a far better choice for 75 Ω stuff, but not the cheap ones you can get from Screwfix that screw onto the end of the cable. Compression types are good but require a compression tool, which you can get cheaply.
Albert H wrote:
Tue Sep 17, 2019 6:36 pm
Low power: 50Ω BNC connectors work well. They also fit RG58 coax easily. These remain pretty low loss up to about 20 Watts.
BNC connectors are also not the impedance that they are supposed to be! Neither the 50 Ω or the 75 Ω versions are 50 Ω or 75 Ω respectively. They are probably good enough for what we're up to, although they are prone to getting wet.

By the time you've got your crimp tool and soldering iron out to put a BNC on, you might as well fit an N connector. You get a sealing gasket (that actually works) for free.
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Re: Thoughts on this please.....

Post by nrgkits.nz » Tue Sep 17, 2019 9:15 pm

Using a 7/16 DIN is a much better option if you’re running high power more than about 800W, these connectors are used on commercial equipment and will comfortably handle upto 5kW depending on the manufacturer and the quality of connector. For even more power use a 7/8 EIA.

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Re: Thoughts on this please.....

Post by XXL » Tue Sep 17, 2019 11:37 pm

May aswell just solder the cable straight to the board...

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Re: Thoughts on this please.....

Post by thewisepranker » Wed Sep 18, 2019 8:59 am

SMA is also very good, small and cheap.

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Re: Thoughts on this please.....

Post by Albert H » Fri Sep 20, 2019 11:14 am

thewisepranker wrote:
Tue Sep 17, 2019 8:39 pm
Before I start, I completely agree that the SO-239 and PL-259 are terrible and not fit for purpose.

"Belling Lee" or "IEC 61169-2" connectors are worse at 75 Ohms than PL-259s are at 50 Ohms. On the face of it, you might think that they are pretty cock on 75 Ohms when you look at the dimensions:
The Belling Lee connector isn't as bad as you suggest in practice. I've measured the impedance discontinuity, and the reflected energy from the discontinuity, and at the kinds of RF levels I use at UHF, they're pretty much insignificant. They also have the advantage that the connectors are cheap and widely available, and if you're putting link receivers and aerials in vulnerable places (like up tower blocks) they're a great choice. They also have the advantage that they're physically incompatible with all the other connectors on the gear so the brain-dead DJ clowns trying to install them can't cock up the plugging!

I NEVER crimp on BNC connectors (except years ago on Ethernet cables)! The ones I used had a soldered centre pin, and the braid was twisted into the (threaded) sleeve. They also had a heat-shrink boot over the cable entry, which gave some measure of waterproofing, but you're right - liquid ingress is a problem with them. Again, the BNC was used to prevent stupid mis-plugging!

These days, we use N-types for higher powers, and SMA / SMB for low power. These appear to be the best of the fairly poor choices that are widely available.

Westinghouse / GE, Marconi, and Continental (amongst others) used to supply their own proprietary versions of the N-Type at very big diameters. They were a nightmare to fit, but worked very well at high power (in the many kilowatt range). However, I've seen one of these large connectors actually explode when it had been poorly installed (by a Marconi "engineer") - it hot-spotted inside and at the power level it was supposed to convey to the aerial feeder, the failure was spectacular!

Back in the dim 'n' distant past, we used to build little VHF rigs inside the antenna support mast! We found that we could virtually eliminate feeders altogether, and just have power and modulation sent up cables inside the mast. The biggest rig I recall building in this way was about 60 Watts, and the antenna was a twin-J. The whole assembly was perfectly waterproof, the pole provided a wind-cooled heatsink, and it was difficult to spot unless you knew what to look for. The very first of these was about 2 Watts, and was on my rooftop in B***a for years. It gave surprisingly good coverage for its low power.
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Re: Thoughts on this please.....

Post by thewisepranker » Fri Sep 20, 2019 2:27 pm

Here's the test results of a cheap pair of IEC to F adaptors that I found laying around.
20190920_123938.jpg
IEC 61169-2 Pair.png
Or if you prefer, log frequency:
IEC 61169-2 Pair (log).png
20190920_125210.jpg
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