Using 2 dipoles instead of 1.

OldskoolPirate · Post by **OldskoolPirate** » Fri Mar 17, 2017 1:21 pm

Can anyone tell me why most London pirates choose to use 2 double stack dipoles instead of just 1. Especially when it's just more equipment to loose. Iv spoke to some people about it but I'm just getting Chinese whispers, nothing concrete and nothing super positive towards it. Also heard that it becomes directional when using 2.

Anyone got some info on this ? Is 2 really better than 1. The gains from them would still only be 1 if omnidirectional.

thewisepranker · Post by **thewisepranker** » Fri Mar 17, 2017 1:58 pm

It doesn't become directional (unless you use the mast as a crude reflector - this requires critical spacing), rather it changes the radiation angle so that you send less of your signal up towards The Moon. The main point, though, is that it effectively doubles your output power.

Albert H · Post by **Albert H** » Sat Mar 18, 2017 3:41 am

There's a very old saying - "The cheapest power amplifier is a better aerial!". A single dipole tends to have a high angle of radiation, so you're broadcasting to aircraft and the space station instead of your target audience. Your ideal radiation characteristic is a sort of doughnut shape, with the least signal going upwards, and very little going straight downwards as well. Stacking a pair of dipoles puts twice as much of your signal into the area where it can be received. Much less of it goes straight upward, and you can actually achieve a better match than you can with a single dipole.

You have to use a proper phasing harness, made from 75Ω coax. You have to calculate the correct lengths for the coax and for the spacing between the dipoles. The dipoles also have to be the correct lengths. There are on-line calculators for all these. The lower dipole MUST be at least 3m (one wavelength) above the supporting structure, so you're going to need a big mast.

You can make the stacked dipoles directional, by standing them away from the mast by the exact distance calculated with a Yagi calculator. This will turn your two dipoles into stacked "H" aerials, and if correctly constructed and aligned, it will give close to four times the transmitter power into the target area - the radiation characteristic looks like a half-doughnut!

Back in the 80s, I got signals with "local" field strength into the more northerly parts of London from blocks way down in the south! I used stacked "H"s and 125 Watt rigs. The ERP was close to 500 Watts, and the improvement in field strength when we used a better aerial was the difference between mono and stereo in the target area. The frustrating thing about the coverage was that down at river level in central London, the signal was only usable in mono - the height of the site was actually working against us!

OldskoolPirate · Post by **OldskoolPirate** » Sat Mar 18, 2017 11:31 pm

So what about a quarter wave ground plane ? The signal is pushed up, does this mean that even more power is wasted up in the sky ? Also I don't get why it's bad to radiate down. Surely that is where you want it?

Albert H · Post by **Albert H** » Sun Mar 19, 2017 3:43 pm

A ¼-wave vertical with groundplanes does radiate quite a lot of the power upwards - as you go towards the ideal (0.64-wavelength), the radiation flattens out, less goes upwards, so there's gain into the target area.

You want as little of the signal as possible going into the block - that's why downward radiation isn't good. I used some aerials that were really weak under the block, but huge from a mile or so away. That confuses the OFCOM numpties since they can only get to within about ½- mile of the site with their Doppler DF gear, then they rely on looking up at rooftops for aerials that "shouldn't be there". Put up an aerial that looks like a CB job with a weak signal below the block, and they'll miss it for ages! It also confuses rig thieves!

THE GOVERNOR · Post by **THE GOVERNOR** » Wed Mar 22, 2017 7:07 am

A pirate in London loves a CB aerial, they have only gone and put it next to a Tetra base station

I'm impressed by the coverage

Albert H · Post by **Albert H** » Wed Mar 22, 2017 7:33 am

It won't actually be a "CB" aerial - it might look like one, but you'll find that it's probably a ⅞-wave (that's what I built a couple of times for a London station). You're right - mounted high enough and in the clear, it's a very efficient aerial!

OldskoolPirate · Post by **OldskoolPirate** » Wed Mar 22, 2017 9:45 am

So why not just use a quarter wave ground plane ? Up and towards the horizon is surley what you want right ?. Also every time I use a ground plane it seems to work slightly better than a dipole. The signal is not so choppy. This is tests done with 5w clear line of sight the whole way.

radio-berlin · Post by **radio-berlin** » Wed Mar 22, 2017 10:19 am

OldskoolPirate wrote: ↑Wed Mar 22, 2017 9:45 am So why not just use a quarter wave ground plane ? Up and towards the horizon is surley what you want right ?. Also every time I use a ground plane it seems to work slightly better than a dipole. The signal is not so choppy. This is tests done with 5w clear line of sight the whole way.

As albert said above, you can use a quartwave ground plane but you are radiating some of your power upward, Why do you want to go up as nobody in the sky is listening to you... unless of course your on low ground.... London is almost sea level hence why dipoles and groundplanes work so well in london as all the power is focused in the area. A grondplane is great on a tower block as less power directly below it driving the residents crazy.
The Only way I can try and descibe it to you is imagine a torch beam with a 5 watt led, if you have a wide beam on the front of the torch it will produce light up down and outward but not as far outward as a narrow beam.... Now if you focus that beam to a narrow beam, you will hardly have any light below the torch or above it but you will have a light beam travelling a lot further away from the torch than the wide beam.. That was a really shit way of describing it but you get the picture if your trying to understand it.

There are pros and cons of each type of aerial and there is no answer to which aerial is better to use as it all depends on your location.
If your down in the hills then a narrow aerial such as 5/8 3/4 etc will just push power into the side of the hill, if your up on a mountain theres a chance a 5/8 3/4 may skim completely over a town located near by but hit a town 50miles away clear as hell . so in this instance a dipole may work better as your power is concentrated in a closer proximity, but depending on the land again a 5/8 may work better depending on where you are trying hit. There is no answer.
Gong further north of london upto milton keynes the land is almost flat so a 5/8 or 3/4 absolutly thumps for miles and miles and miles up north, yet going south only clears a few miles because of the land height. Again another shit example but you get the picture.

nrgkits.nz · Post by **nrgkits.nz** » Wed Mar 22, 2017 10:58 am

I use a folded dipole from one of the highest sites above Auckland close to the TV tower, about 330m above sea level. I get reasonably good feild strength directly down through the valley below. I'd use stacked yagis, as many as I could fit on the mast, but this isn't permitted for LPFM. eirp is limited to 1watt without a license and they also take into account antenna gain.

sinus trouble · Post by **sinus trouble** » Thu Mar 23, 2017 11:00 pm

Quarter wave antenna seem ideal for 'Cabbies' They have an excellent groundplane (the car) and cos of their low elevation, upwards and outwards makes sense!
I kinda understand antenna and the more radiating elements you can accommodate? the better? with the band in question, its obviously unfeasible to use more than a double!
Interestingly as Mr NrgKitz mentioned Yagis? I have seen UHF antenna in a quad stack folded dipole configuration, but each at 90 degrees apart around the mast??? Whats that all about?? lol

yellowbeard · Post by **yellowbeard** » Thu Mar 23, 2017 11:33 pm

The reason for putting UHF aerials around a mast like you describe is because the mast itself acts as a bit of a director, so the antenna is not truly omnidirectional any more if the dipoles are on the same side. It is only a DB or two, and most users arrange to point a stacked dipole at where they'd like a slightly stronger signal.

Albert H · Post by **Albert H** » Fri Mar 24, 2017 3:36 am

You can do some really clever stuff with aerials if you know what you're doing. I wanted a moderately concentrated signal into a couple of parts of LA, so I used Yagis from up in the Hollywood Hills. My 40 Watts of transmitter power was actually close to 200 Watts ERP! I got a good signal into the parts of town that mattered to us and didn't waste power by broadcasting to areas that wouldn't be interested in a white Rock station!.

bopper1968 · Post by **bopper1968** » Mon Apr 17, 2017 12:47 am

Is there any sites that I can learn more about what is the best antenna etc. I have 3 x 200w transmitters that are longing to be used again. One in Scotland and one in Liverpool to start.

thewisepranker · Post by **thewisepranker** » Mon Apr 17, 2017 1:00 am

This site is a good example. Just tell us what you're trying to do and we can advise.

MC Spanner · Post by **MC Spanner** » Mon Apr 17, 2017 1:09 am

You can learn a lot from ham radio sites - they love constructing antennas. dxzone.com, eham.net, qrz.com for starters, there's plenty of discussion on there. Have a look for antennas for 2m and 4m, that should give you some ideas in the right ball park. Then, there will be people on this site who can advise further.

Using 2 dipoles instead of 1.

Using 2 dipoles instead of 1.

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