From To Received Bytes Subject
G0FTD ANTENA @ WW 2004-05-27 18:44 9705 Indoor wire antennas 160m - 6m

From: G0FTD@GB7WIG.#34.GBR.EU
To  : ANTENA@WW


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##This bulletin is updated from the previous bulletin I sent a few days ago##
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Here's a real cheap antenna to make from 160m - 6m and WORKS !

PREAMBLE:

It was developed simply beacuse of my own personal circumstances which 
meant I often had to play radio from restricted QTH's. My employment in
discrete antennas sure helped me out, as not only did I have demanding 
customers for discrete antennas but I find myself missing amateur radio 
when operating from hotel rooms abroad or "digs" in the UK ! Of course I've 
done all the large antennas from home but it's too easy and I want to have
a bit of a challenge from building COMPLETE indoor stations totally indoors
and so the following has been in use for the past few years by myself almost
daily.

The antenna is nothing more than a simple 2.4 metre square loop "drawing 
pinned" to the internal brick wall of the spare bedroom. Yep thats right
the INSIDE wall of the spare bedroom - ideal for flat dwellers, hotel rooms
or whinging neighbours !

The loop has a simple switched inductance at the top of the square loop
and uses a simple coaxial stub to tune the antenna. An additional variable 
capacitor placed across the feedpoint can be used to fine tune the 
resonance of the antenna. The basic configuration is shown below.

                             -------/\/\/\/\-------
                            |                      |
                            |                      |
                            |                      |
                            |                      |
                            |                      |
                            |                      |
                            |                      |
                            |                      |
                            |                      |
                            |-----------O----------| 

Where /\/\/\/\ represents a switched inductance.
Where O is the feedpoint plus coaxial stub.

At point O the coaxial stub can be replaced by a good quality ATU, preferably
one which doesn't use toroids !

A balun is not required and at the most a simple choke style balun made from
about 6 turns of coax and about 6 inches diameter can be used to attenuate
radiation from the feeder cable. Good quality ATU's are expensive and often
cost around 300 pounds sterling so to cut costs the simple coaxial stub can 
be use to utilise the self inductance and capacitance to from a tuned
matching circuit to provide a 50 match to the amateur tranceiver. It is also
possible to connect a variable capacitor at either the beginning or the end 
of the coaxial stub in order to provide across band fine tuning.


The antenna is fed by simple 50 coaxial line.

Here's the description of the antenna at various bands.

160m:
Use 100H inductance at the top.
A coaxial stub which is open ended RG58 style coax is 48cm long.
Open ended means just dangle a piece of coax at point O with braid to one side
and centre to the other. The end is left "open" or with a simple variable
capacitor for fine tuning. Results have been reasonable for local working
but not amazing. Generally I qso 50 miles or so.


80m:
Using ZERO inductance and 5 watts I have made plenty of UK/EU qso's with
this antenna. Obviously larger antennas are better but expect your signal
to be about 2 s-points down from stations using full size antennas and 
assuming the same power output.

40m:
Using 10 watts I have had some QSO's around the UK but I am not keen on this
band so my experience is limited. Zero inductance and a 39cm stub is
required.

30m:
THIS BAND AND QRP IS BRILLIANT! 5 watts rarely fails to get a QSO. I have
only operated in the daytime when I have numerous QSO's around europe with
just 5 watts with NO hassle. Just uyse a 30cm stub and zero inductance.

20m:
Numerous european and north american qso's made in the direction of radiation
of a normal quad loop. Use 15H of inductance for best results.

17m:
Numerous north americans, the middle east on 1 watt and Russian ragchews
are easy. Due to the radiation pattern of the antenna in my QTH I am best
placed for north american and middle eastern QSO's.

15m:
All over north america, the carribean and some south americans with 10 watts.
1 watt to the Lebanon and recently my first call was answered by the 3B9C
expedition using 25 watts of ssb. Countless europeans of 5w cw. North
americans worked by the truckload. Brazil also worked. Use zero inductance
anbd 6cm of coaxial stub.

12m:
Plenty of north americans and europeans. Use 9cm of RG58 stub and zero
inductance for best results.

10m:
Plenty of north american, the caribbean and the middle east as well as
europeans on 29MHz FM and I only used a max of 25 watts. Brazil also worked.

Remember the quad loop up on these freqs is like a horizontal dipole and
as such is directional.

Use 15cm of coaxial stub and zero inductance.

HOWEVER we can perform a useful trick here - use 60H of inductance and
you can rotate the radiation in the opposite direction and with vertical
polarisation ! A rotatable indoor dipole without rotating it ! This time
you use 60H of inductance at the top of the loop and approx 20cm
of open ended coaxial stub.

I've worked gawd knows amounts of north americans, south americans,
central americans, the middle east, europeans and Russia and south african
stations with only 10-20 watts ssb.

Remember all of the above has taken place outside the sunspot maxima.

6m:
In the summer sporadic E season it is typical to be able to work distance
about 1500 miles. With only 5 watts cw/ssb I have had plenty of qso's at this
distance with ease in the direction of the loop, which is in the same
direction of a normal quad loop. I don't frequent 50Mhz all that often
but it's pleasing to work exactly the same typical distances as every one else
with this silly antenna !

NOTES:
This antenna is like any other antenna - it's just a resonant circuit
that radiates. The loop is the inductance and all you need to do is add
the required capacitance to it to for a tuned and radiating circuit.

Rules of thumb are that the smaller the loop the more capacitance you need
to resonate it. When tuning a coaxial stub you simply snip off 5mm at a time
to provide the required match. Circumstances vary ! So be prepared for
slight differences to stub lengths etc. It is perfectly in order to have
a reasonable variation in loop length and adjust sub length/variable
capacitance in order to suit.

All polarisation is horizontal except for 160m where it is vertical OR
on 28Mhz using 60H inductance it also becomes vertical polarisation.

This antenna as described IS in use by G0FTD every day and provides many
happy hours of operating - so I know it works.

With the restrictions placed upon so many of us these days it really is 
pleasing to report a real antenna that allow just about anyone to play
amateur radio with some sense of normality. Youngsters with parents can
put up this antenna in their bedrooms, pensioners can (and HAVE) used 
this antenna in retirement homes and restricted accommodation, students
and flat dwellers can use it to at least continue their chosen hobby.


Noise is a big problem on receive and is often very restrictive. Sorry folks
but it's a problem we ALL suffer from. Apart from operating at night when 
hopefully the local TV's have been turned off there's not a lot we can do.
But it's strange that a 2am I can call CQ at 100 watts and observe the QRM
disappear within 10 mins ;-) <cough cough> !


CONCLUSION:
Amateur radio IS possible under the most extreme circumstances so long as you
don't expect all freqs to be 100% qrm free on recieve and that you'll have
to expect your signal to be a few s-points lower. In practice it's not
really a problem and then it pays to think that if your using a bit of
"damp string" and a few watts your acheivement is damn good compared
to all the sillywotsits who spent 1000's or euros/pounds/dollars and massive
effort for towers/kilowatt amps/expensive aluminium tubes to make yagi's
blah blah.


It's all relative folks - so ENJOY amateur radio under your OWN circumstances
and stop feeling left out !

A BETTER LF ANTENNA.

I have also tried a 4.2m x 2.4m loop with excellent results on the LF bands.

It seems to outperform the previous antenna on the LF bands by quite a large
margin. The antenna is a corner fed loop as shown below.

                       ----------------------
                       |                    |
                       |                    |
                       |                    |
                       |                    O = Feedpoint
                       |--------------------|


I have NOT tried coaxial stubs with this antenna.
I have only use an atu for matching.

However I have used computer modelling to assist me with a high degree of
accuracy with the previous antenna so here are the recommended starting 
points for using open ended coaxial RG58 stubs.

160m:
70cm stub

80m:
5cm

40m:
19cm

30m:
38cm

20m:
48cm

17m:
6cm

15m:
7cm

12m:
12cm

10m:
5.5cm

6m:
8.5cm


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This simple ascii article can be used in ANY non profit magazine/newsletter
or amateur radio website without charge blah blah - but it would be nice
to know who's using it and acknowledgement to Andy G0FTD is given.
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Long live HF amateur radio.


- Andy -