I live in the top flat of a building with a flat roof about 24' (7.5M) x 21'
(6.5M) and 30' (9M) above ground. I need an 80M antenna to operate locally
around the SE part of England. I have limitations on what I can erect.

I thought of a horizontal loop about 3' (1M) above the roof, possibly with
90uH to 100uH in each of the two side legs to help it resonate on 80M. I'm
not very good at modelling and could not work out how I enter a reactance
across the feedpoint to balance out the large negative reactance there. The
FFT plot with EZNEC looks promising.

Can someone help me to work out how to feed this antenna and what reactance
I should add at the feedpoint.

I have some options segarding the feeder and ATU. I have an SGS SG-230 ATU
which I could mount near the loop or an LDG Z-11 that I could mount near the
transmitter with a balun (I have several from 1:1 to 16:1) and balanced feed
to the antenna. My preferred feeder is 50 or 300 ohm balanced as they are
easily concealed or alternatively coax feeder and balun at the antenna.

Thanks for any help.
Richard (Dick) G4BBH

If you're only interested in 80m, get a good split stator loop tuning
capacitor and put it opposite the feedpoint in the loop. Make the loop
out of the fattest conductor you can find.

It'll basically be a big halo antenna for 80m. (maybe you could call
it a magnetic loop.. i don't know if people ascribe a particular
fraction of a wavelength limit on magnetic loop sizes)..You'll need a
motor drive for the cap, because as modeled with 3/4" copper tubing and
3-j1200 ohms opposite the feedpoint, the 2:1 SWR bandwidth is about
15kHz.

Radiation resistance is 1.85 ohms or something like that, so you'd need
low loss connections for all the wires and a welded capacitor... if
you do it all right, though, you'd have a good efficient radiator for
local communications.

Just a thought.

I think this would give you better performance than trying to
inductively load the legs.

73,
Dan

Dan

Remember that this has to be a 'stealth' antenna as I live dead center in
the town with a lady councillor opposite, the town council office 75 yards
away and town hall about 150 yards away. This rules out low loss copper
pipe etc. Most of the small 'magnetic loop' antennas are intended for
vertical use which would also stand out like a sore thumb here. The
advantage of the wire loop is that many buildings here have wire around the
roof to deter seagulls, that means my wire would not be conspicuous.

When I modelled my 21' x 24' horizontal square loop with loading coils of
about 90uH in each side leg I got a feed impedance of over 300 ohms but
with -j5000 or thereabouts. The swr curved dipped nicely at 3.65 MHz to a
manageable level if I used a 4:1 balun and 300 ohm line feed. I reasoned
that as the natural loop resonance was between 20M band and 30M band I could
link out the coils and operate those bands too, although that would means
climbing up a ladder out of the rooflight rather more often than my old
bones like.

Richard (Dick) G4BBH

ferrymanr wrote:
Dan

Remember that this has to be a 'stealth' antenna as I live dead center in
the town with a lady councillor opposite, the town council office 75 yards
away and town hall about 150 yards away. This rules out low loss copper
pipe etc. Most of the small 'magnetic loop' antennas are intended for
vertical use which would also stand out like a sore thumb here. The
advantage of the wire loop is that many buildings here have wire around the
roof to deter seagulls, that means my wire would not be conspicuous.

When I modelled my 21' x 24' horizontal square loop with loading coils of
about 90uH in each side leg I got a feed impedance of over 300 ohms but
with -j5000 or thereabouts. The swr curved dipped nicely at 3.65 MHz to a
manageable level if I used a 4:1 balun and 300 ohm line feed. I reasoned
that as the natural loop resonance was between 20M band and 30M band I could
link out the coils and operate those bands too, although that would means
climbing up a ladder out of the rooflight rather more often than my old
bones like.

Richard (Dick) G4BBH



In 'G' land what is the concern regarding public officials and radio antennas?

Here in W/K land public officials and radio antennas are not a concern. Public
safety issues are a matter of local building codes. Private contracts, known as
CC&Rs, are a problem.

ferrymanr wrote:

Remember that this has to be a 'stealth' antenna as I live dead center
in the town with a lady councillor opposite, the town council office 75
yards away and town hall about 150 yards away. This rules out low loss
copper pipe etc. Most of the small 'magnetic loop' antennas are
intended for vertical use which would also stand out like a sore thumb
here.

It's surprising what can be done, with a bit of determination that you
*are* going to get on the air.

I know a ham who lives in a Listed building, which means he can hardly
even paint a door without consulting the local Council's Conservation
Officer. To make life interesting, he lives right next to the Council
offices, and there's a perfect view into his backyard from the
Conservation Officer's desk! Now that's what I'd call really living with
restrictions... yet he has still found ways to get out well on 80m.

(His antenna would be very unlikely to suit your situation, so there's
no point in describing it in detail.)

Always remember that you have rights too. With certain exceptions, most
UK householders have the right to put up a garden shed without asking
anybody's permission at all. A surprisingly large shed, too - with a
pitched roof, the ridge could be up to 4 metres high.

Check how that would apply to you. And then, if anyone objects to you
using a 4m high vertical loop, ask them if they'd seriously prefer that
you build a whole big shed around it?


The advantage of the wire loop is that many buildings here have wire
around the roof to deter seagulls, that means my wire would not be
conspicuous.

That would certainly be another option to explore; but whatever you do,
be sure to make the best use of all your rights.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:
ferrymanr wrote:

Remember that this has to be a 'stealth' antenna as I live dead center
in the town with a lady councillor opposite, the town council office
75 yards away and town hall about 150 yards away. This rules out low
loss copper pipe etc. Most of the small 'magnetic loop' antennas are
intended for vertical use which would also stand out like a sore thumb
here.

It's surprising what can be done, with a bit of determination that you
*are* going to get on the air.

I know a ham who lives in a Listed building, which means he can hardly
even paint a door without consulting the local Council's Conservation
Officer. To make life interesting, he lives right next to the Council
offices, and there's a perfect view into his backyard from the
Conservation Officer's desk! Now that's what I'd call really living with
restrictions... yet he has still found ways to get out well on 80m.

(His antenna would be very unlikely to suit your situation, so there's
no point in describing it in detail.)

Always remember that you have rights too. With certain exceptions, most
UK householders have the right to put up a garden shed without asking
anybody's permission at all. A surprisingly large shed, too - with a
pitched roof, the ridge could be up to 4 metres high.

Check how that would apply to you. And then, if anyone objects to you
using a 4m high vertical loop, ask them if they'd seriously prefer that
you build a whole big shed around it?


The advantage of the wire loop is that many buildings here have wire
around the roof to deter seagulls, that means my wire would not be
conspicuous.

That would certainly be another option to explore; but whatever you do,
be sure to make the best use of all your rights.



I would transmit on an efficient short vertical, and use a magnetic loop
on Receive. For that matter any low noise receive antenna like a Flag
or K9AY. I would also look at something like the Welbrooke Active loop.
I wont get into the debate of "magnetic" and noise! Just go to
Welbrookes web page and read some of the reviews from a EMC expert on
noise and comparing the Welbrooke to 20,000 Dollar R&S EMC antennas E
and H plane antennas.

If W8JI and others can work DX on 160meters from an efficient mobile
antenna, you should have no trouble getting out. There was a guy on 80
meters from the west coast K6MB who used to beat stations with dipoles
in the 80 meter DX window with a coil loaded mobile antenna!

Dont believe all the high angle low angle nonsense, the difference in
real terms amounts to one deep fade. I would never feel at a
disadvantage running a vertical versus a low dipole.

There was a interesting article in QST by Some PA hams who used a R&S
super resolution DF antenna and measured incoming arrival angles across
Europe, oh how i would like to have one of those setups! They use cross
loops too.

Zen