The Inverted L and FCP Counterpoise
Living in a typical Scottish house , I have a typical Scottish garden ie. its wee !
So getting on Top band (1.8mhz) is not that easy when my back garden is only about 12 metres deep by about 10 metres wide so not a lot of space for a half wave dipole which is 80 metres long and should be at least 20 metres high.
An easy solution is to install an Inverted L which is a quarter wave aerial known as a Marconi type aerial devised and designed by Marconi the father of radio as we know it, way back in the early days. It is 42 metres long and run it as high as you can straight up, and then bend the rest horizontal for as long as required.
A quarter wave vertical has an impedance of 50 ohms which is a perfect match for modern radios so the inverted L type while not vertical should be close enough in impedance that an aerial tuning unit will get a good enough match to 50 ohm.
Getting that inverted L to be efficient enough is a major problem however ,the inverted L needs something to push against to launch its RF.
Putting in a few grounds rods is mentioned in various literature as a way of getting a decent ground for the aerial, but there are problems with that approach, the first being is it electrically safe to install ground rods in your garden without inadvertently creating a hazard !.
In one of my previous posts I mentioned the hazards of exporting your household earth via a coax feeding a grounded aerial, Here in the UK with a conventional type supply and especially a PME supply (where exporting the earth is forbidden under electrical regulations) you are taking a chance that you will never loose your incoming neutral and hence create a shock risk in your home. Please read posts further back for the types of supplies in the UK.
The second problem with ground rods is you have no control over how good your ground plane is and in any case in a small garden there is not enough room to run proper ground radials so a better approach is to install an above ground counterpoise .
The inverted L itself.
So as you can see from the above sketch I still dont have enough room , so with 42 metres to loose I fold the top horizontal section back towards the mast, my maximum height is at about 12 metres but if you can go up higher all the better as this will reduce whats left to bend horizontal
The Inverted L is attached to the top of the mast, goes towards the apex of the house and comes back towards the mast. I hear you cry ohh that will cancel its self out, yes there will be cancellation but if I did not put up the aerial I would not be on the air at all !
I have found you need to keep a least a metre between the top horizontal wire and the wire underneath to get a match.
The inverted L is fed at ground level against a counterpoise
The Counterpoise
The FCP: A 160 Meter Counterpoise for a Postage-Stamp Lot
Guy Olinger, K2AV / k2av@contesting.com
https://k2av.com/Olinger_NCJ_article_on_FCP.pdf
https://k2av.com/
So the above link will take you to the original article from NCJ magazine, where the magic of the folded counterpoise was first published and I can attest to its effectiveness.
The folded counterpoise is a 5⁄16 λ 50 metre long wire folded back twice and takes up only 20 metres in length, in my case this is in a U shape as it runs along the back fence and up towards the house about 4 metres on each side. I have mine mounted at the 6 feet level and its works as described.
The FCP is made to work with a simple isolation transformer (you can make this !).
A pair of T300A-2 powdered iron toroids are taped together and the winding's are wound round to create the transformer
So getting on Top band (1.8mhz) is not that easy when my back garden is only about 12 metres deep by about 10 metres wide so not a lot of space for a half wave dipole which is 80 metres long and should be at least 20 metres high.
An easy solution is to install an Inverted L which is a quarter wave aerial known as a Marconi type aerial devised and designed by Marconi the father of radio as we know it, way back in the early days. It is 42 metres long and run it as high as you can straight up, and then bend the rest horizontal for as long as required.
A quarter wave vertical has an impedance of 50 ohms which is a perfect match for modern radios so the inverted L type while not vertical should be close enough in impedance that an aerial tuning unit will get a good enough match to 50 ohm.
Getting that inverted L to be efficient enough is a major problem however ,the inverted L needs something to push against to launch its RF.
Putting in a few grounds rods is mentioned in various literature as a way of getting a decent ground for the aerial, but there are problems with that approach, the first being is it electrically safe to install ground rods in your garden without inadvertently creating a hazard !.
In one of my previous posts I mentioned the hazards of exporting your household earth via a coax feeding a grounded aerial, Here in the UK with a conventional type supply and especially a PME supply (where exporting the earth is forbidden under electrical regulations) you are taking a chance that you will never loose your incoming neutral and hence create a shock risk in your home. Please read posts further back for the types of supplies in the UK.
The second problem with ground rods is you have no control over how good your ground plane is and in any case in a small garden there is not enough room to run proper ground radials so a better approach is to install an above ground counterpoise .
So as you can see from the above sketch I still dont have enough room , so with 42 metres to loose I fold the top horizontal section back towards the mast, my maximum height is at about 12 metres but if you can go up higher all the better as this will reduce whats left to bend horizontal
The Inverted L is attached to the top of the mast, goes towards the apex of the house and comes back towards the mast. I hear you cry ohh that will cancel its self out, yes there will be cancellation but if I did not put up the aerial I would not be on the air at all !
I have found you need to keep a least a metre between the top horizontal wire and the wire underneath to get a match.
The inverted L is fed at ground level against a counterpoise
The Counterpoise
The FCP: A 160 Meter Counterpoise for a Postage-Stamp Lot
Guy Olinger, K2AV / k2av@contesting.com
https://k2av.com/Olinger_NCJ_article_on_FCP.pdf
https://k2av.com/
So the above link will take you to the original article from NCJ magazine, where the magic of the folded counterpoise was first published and I can attest to its effectiveness.
The folded counterpoise is a 5⁄16 λ 50 metre long wire folded back twice and takes up only 20 metres in length, in my case this is in a U shape as it runs along the back fence and up towards the house about 4 metres on each side. I have mine mounted at the 6 feet level and its works as described.
The FCP is made to work with a simple isolation transformer (you can make this !).
A pair of T300A-2 powdered iron toroids are taped together and the winding's are wound round to create the transformer
The above illustration is from Guy Olingers website at https://k2av.com/ which I urge you to visit.
So how well does it work ?, well previously my top band efforts were hopeless.
Now my 2:1 swr points are 1.866MHz and 1.966MHz so it has 100 kHz bandwidth and the band edges are 3:1 which are easily handled with an ATU.
Ive had this set up now for about 3 years , it still works fine and I run mainly QRP power at 5 watts or less and during the recent cq ww multi band contest my QRP efforts on top band snagged me 58 contacts including many Europeans , Asiatic Russian and a solitary Canadian station.
I can also run 400 watts if required....but having that much RF on a relatively low aerial in such a small garden can cause its own problems because of the strong RF field.
But success on transmission can have unexpected results on reception. On top band because of the high noise level I also use a K9AY to receive and because of the proximity of aerials when running relatively low power I can get RF feedback directly on my receive coax, its another issue of a small garden.
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