Aerial situation for low power directional

[Albert H]

It was mentioned a mast being used as reflector, the way described does that mean the dipole would look like an H ((((

Suppose it has bit longer the reflector, I had one used to put behide a dipole in the loft it did work to a degree, it was mostly still omidirectional. The mast will be a longer than a reflector to get it off the ground, so I can’t see how it would do alot.

Northeast - the reasons that it didn't work for you are:

1. It was in a loft. You're going to get reflections off everything around your aerial, your radio energy isn't getting radiated (you'll lose most of it into the roof), and if the roof is wet with rain, you'll get almost nothing radiated!

2. You didn't have the dimensions correct. The distance between the radiating element and the reflector is crucial. If you get it wrong, your SWR will be abysmal, sending power back down the feeder. The impedances (real and virtual) of the aerial will also change - possibly quite radically - and it just won't work!

NEVER put a transmitting aerial inside a loft - you'll waste at least 90% of your expensively generated RF energy!

Mongo82 - You're right. A two-element Yagi (a radiator and a reflector) does look like an "H".

In answer to your question about the PL259: A friend of mine used to call them "audio plugs" because they're so bad at VHF frequencies! If you use a TDR (or a VNA) to look at the feeder from the output of the final PA filter, all the way to the aerial, you'll see that the SO239 / PL259 combination gives a big disconinuity in the trace. This shows that the socket and plug combination doesn't work very well, and causes reflections of its own.

Better connectors include the BNC (be careful to use the right impedance version - they come fractionally differenly sized for 50Ω and 75Ω) and the "N"-type. There are several other (SMA, SMB spring to mind) that also work well.

You may be surprised to learn that (at low power) even the humble RCA / Phono plug and socket outperform the PL259!

Unfortunately, fitting SMA plugs need the right tools, so most people use ready-made cables. They have the advantage that they provide a good match, can handle a few Watts, and give a physically robust connection.

The BNC is the one I'd usually favour for up to (perhaps) 20 Watts. You don't need any special tools, and the provide a reliable, low-loss connection. They're also easy to connect and disconnect, and you can get ones with the right collar internal size for your RG58 (or RG59 if you're going for a 75Ω system). I can't vouch for other suppliers, but RS Components used to supply a little pamphlette with their BNC connectors, showing how to fit them correctly!

[EFR]

For SMA powerhandling, it does 1.5kW on HF all day long, but 300W on VHF and it melts.

[Albert H]

We use SMA for RF connections between boards inside gear. They're robust, and tend not to come apart even with severe vibration.

Link receive aerials are always 75Ω Belling-Lee Type (TV aerial download type). Again, they provide a reliable connection , low loss, cost next to nothing, and can't get muddled up with any other kind of connector. These were especially good when using UHF TV Yagis for links, since you can use the brown-coated "Low-Loss" TV downlead.

[FMEnjoyer]

Any metal including your mounting pole within some meters of an antenna becomes an integral part of your antenna system.

It will form a reflective or directive element and there will possibly be parasitic capacitance.

It is in your reactive near field which means as Albert mentioned will affect radiation pattern, angle of radiation and skew the directivity.

I used to use broomsticks as booms for dipoles as I never trusted the metal boom arm. Though was not savvy enough to consider the mounting pole which is of course not easy to make from wood or use a balun. We all learn more as you go.

A GPA remains a very simple and effective antenna. You can make them from brown wire and set them in nature barely seen, you would have to be really looking.

They will sway around plenty but they are quite unfussy really, dangling legs without support still matches well, make from copper and the match is extremely efficient. Stick one on flag pole and raise it in hours of darkness and you will be much better off than in the loft.

Lofts work though of course you will lose some energy and your pattern will be messed up but you will be on and very discretely so. Not ideal but if it is all you can do.

You have to work with your on situation with antennas, some people don't have a garden and some don't have trees, some nosy neighbours or are overlooked etc. You can only go so far with what you have.

I am happy to just read and write about radio these days, I prefer listening and being on here, if you want to come on and all you have is a loft a copper wire GPA made from an SO-239 will work very well SWR 1.3:1

Directionality becomes complicated for a beginner. GPA with all 3 or 4 ground plane legs broadly bent in one direction will skew your radiation a little maybe 1-2dB in the direction the legs are going. By far the easy way to get easy match and some directionality without a tonne of faff.

[radionortheast]

An outdoor aerial is better but it was all that was availble at the time, it was mostly used for receieving anyway, when there wasn’t much there anymore went back to an omi. Theres noway to tell the spacing was correct anyway, when a 4 inch hole was found, omi aerials could go out through it. When I had a dipole in the loft, either had a choke, or pawsey stub, so there was no backwards radiation, in the time the 4w did get out well been able to hear it on a car radio afew miles away, like I said in the other thread could hear it 20kms away, back in the exciting days, the place I received it at has since been pulled down, it was not an actual radio station, just music I liked on it.
If it had been something that hadn’t worked we couldn’t drive around listening to on a car radio I wouldn’t of bothered.
I do think what Albert is saying is mostly spectulative, perhaps maybe thinking of new builds, were theres likely to lots tin foiled backed insolation. All the 3 lofts i’ve been in have felt, then tiles, it will get out better if its outside it will be higher up, getting someone to install that aerial, what happens if you have to move frequencies? problematic radiation back down the feeder. Even if you can get your own aerial outside, if its safe to do so, you’ll have to make sure it's not going to get blown down by the wind, having to avoid the big bad wolf huffing and puffing, its rain proof and that it is very light weight.
I mostly only use 1w now from the loft, car radios have become less sensitive now, only listen a mile away on a pocket radio, if people want to drive around listening to their own then its the own business, it is part of the past.

[radionortheast]

Obviously theres some advantages to an aerial been in loft, as long as its not like a faraday cage up there, the weather can’t get to it, it can be easyly changed. It won’t be seen, I think the visbilty of an outdoor aerial might be over rated, if its not interfering it won’t bother anyone, unless its on a really long pole, maybe at the end of a road.

They did have a chart on pcs website for a gp antenna at different frequencies, I found this worked perfectly cut the radials to the size on the chart, then cut the top part down. For a lengh of less than 10 meters I used thin 5mm 50 coax, they are not that fussy, you can use wire for the radials too, hold them out with thin plastic, if the wires aren’t that rigid, you can use part of an old folded dipole for the top, the whole thing would be light. Trying to make a more complex aerial like a j you could end up with raidaiton going back down the feeder. I find even with dipoles the lenght is often too long, making chokes, or a pawsey stub, obviously for someone without experience it would hard to do, the need for specialist equipment to set it up, instead of been straight forward. A homemade dipole on a mast will put more weight on one side if its centered fed, weather it is center fed or end fed, will not work without a pawsey stub or a choke, with the gp you already know the angle of the radials should be, theres no choke or pawsey stub, unless your trying to get to work with one radial, then your back to something simliar to a dipole with the bottom part pointing up. https://www.pcs-electronics.com/guides/antennas/

[FMEnjoyer]

An outdoor aerial is better but it was all that was availble at the time, it was mostly used for receieving anyway, when there wasn’t much there anymore went back to an omi. Theres noway to tell the spacing was correct anyway, when a 4 inch hole was found, omi aerials could go out through it. When I had a dipole in the loft, either had a choke, or pawsey stub, so there was no backwards radiation, in the time the 4w did get out well been able to hear it on a car radio afew miles away, like I said in the other thread could hear it 20kms away, back in the exciting days, the place I received it at has since been pulled down, it was not an actual radio station, just music I liked on it.
If it had been something that hadn’t worked we couldn’t drive around listening to on a car radio I wouldn’t of bothered.
I do think what Albert is saying is mostly spectulative, perhaps maybe thinking of new builds, were theres likely to lots tin foiled backed insolation. All the 3 lofts i’ve been in have felt, then tiles, it will get out better if its outside it will be higher up, getting someone to install that aerial, what happens if you have to move frequencies? problematic radiation back down the feeder. Even if you can get your own aerial outside, if its safe to do so, you’ll have to make sure it's not going to get blown down by the wind, having to avoid the big bad wolf huffing and puffing, its rain proof and that it is very light weight.
I mostly only use 1w now from the loft, car radios have become less sensitive now, only listen a mile away on a pocket radio, if people want to drive around listening to their own then its the own business, it is part of the past.

Cheap fibre glass 10-12m flag poles are collapsible, they go up and down in a flash, gaffer tape a wire GPA antenna on, gaffer tape the mast to anything, pick and choose your nights weather wise - who wants to go on every day and get caught anyway ? And one can have fun every now and then if that is fun for anyone these days. GPA a little less susceptible to common mode rf on coax as it has radials, compared to a dipole with no mitigations. I doubt you lose 90pct of power in a loft, unless roof is wet but it is obvious some will be absorbed by walls, contents of loft tiles, wiring, all attenuating your signal. stealthy though.

I agree it is part of the past, a few carry on but it'll all die off soon enough I suspect. Not that i hope that, I wish there were some Veronica Eastbourne's locally to listen to, fact there is there isn't. Cause most people gave that lark up long ago.

Dipoles are best in the bin for beginners messing around, GPA all the way, incredibly simple and light no messing around with baluns and stubs etc. They match by design.

[EFR]

Some new (after 2000 I think?) roofing tiles have copper on them to stop moss grow.

[Mongo82]

Some new (after 2000 I think?) roofing tiles have copper on them to stop moss grow.

Cheers fellas, loving the idea of telescopic flagpole and will definitely build the gpa. Couple of questions.

All i could find were calculations for 144mhz which is a quarter wave.

Do I need to make a half wave for 88 - 108 and if so can someone give me the formula to calculate?

[yellowbeard]

Ground plane calculator:
https://m0ukd.com/calculators/quarter-w ... alculator/

I am surprised nobody said a Moxon antenna, it does about 180 degrees in a cardiod pattern when vertically mounted, and has a useful bit of gain compared to a dipole or groundplane:
https://www.antenna2.net/cebik/content/ ... xpage.html
https://www.antenna2.net/cebik/content/moxon/mox20.html

[FMEnjoyer]

Some new (after 2000 I think?) roofing tiles have copper on them to stop moss grow.

Useful information, never heard or can find anything about that here....just test, if you cannot be heard up the road something is up.
The obvious way is winter is best, dark early, stick a pole up no one knows anything. More wet and wind to avoid though.
Just use your noddle and be sensible.

All in the past now, few are listening.