PA8W Amateur Radio 

Wil, PA8W,  E-mail: PA8W@upcmail.nl         
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Small but effective antennas for the higher HF bands




My present 20m circumference loop. Turning radius 2,10 meter, (6,5ft)!
 I can tune it on 40m, 20m, 15m 
(and 10m with top segment opened)
On 40m it seems to work better for DX than the Inverted V!
 On the left you can just see the wire of the (lowered) Inverted V doublet for 80m and 40m.
When 20m is not open for a longer time, I use the same pole to
push up the feedpoint of the Inverted V to about 12m height.
This way I can work 80m and 40m.
The changeover is done within 5 minutes...

In search of the best small (but effective!) 20m antenna I tried quite a few antenna options using a 12m telescopic glass fibre push-up pole. (from spiderbeam)
This pole is very convenient to put up an antenna at heights that exceed the size of your lot. 
I made some fixing points to my garage, starting off on the flat roof of the attached carport.
The attic of my garage is my shack, and through a little door I can step out of my shack directly onto the carport roof.

So, here I had a quick and easy platform for my antenna experiments.

Additionally, I always use a comparison antenna, since there is no way of properly judging an antenna without having a reference antenna in place. Quickly switching between the two antenna types gives you reliable data about antenna performance on stations at various distances. However, make sure that  the two antennas have the same favoured direction, so your judgement is not blurred by directional properties of one of your antennas.

Over a period of 1,5 years I built and tested quite some antenna types.
The choices I made were inspired by the ARRL antenna book, by the great articles of W4RNL Mr Cebik, and last but not least my own humble opinion. (which I had to adjust a few times...)
Looking back I would have been better off simulating and modeling on the computer first, using MMANA or another nice program. But then, modeling doesn't take in account everything. Reception quality for example is highly depending on where your QRM is originating  from. Is it local, seemingly originating from the horizon?  Is it short distance but ionospheric?
We will not go into that science here but we will just note the differences we meet in our tests.

And another misunderstanding blurred my opinion about several antennas: Lots of antennas are specified or calculated in free space, which of course is not the real world. At least on the higher bands you need a pretty low radiation angle. I now judge 40m antennas at a take-off angle of 30 and 15 degrees, and a 20m antenna at 10 and 5 degrees, modelled at the correct height over ground. This way, the outcome of my antenna experiments show a pretty close match to the calculated data. 
I stopped trying to get an  easy 50 ohm match; the antenna performance is much more important, and a low loss match is almost always possible using a symmetric feedline and tuner.

I've put some data in a table, for easy comparison:
The numbers in the table show the gain in dBi at the specified take-off angle.
The blue row is the currently used 20m loop, top wire at 15 meter height. 
For me, this row is to be used as a reference, since I look for improvement over my existing antenna.

Note that on 28.4MHz (as on 51MHz) the top wire is opened, so at this frequency this antenna
performs as a Bi-Square, and is less than 2,5dB down compared to a full size 2-el Yagi!
 
Also note that even on 40m, this is a very reasonable antenna, less than 2dB down compared to a full size 40m loop. This small difference is almost impossible to notice or prove in real life.

For DX (low take-off angles) height is much more important than size.
And this small and light loop can easily be pushed up a few extra meters, and still be rotated by hand if necessary.


 Frequency ----->
take-off angle ----->

7.1MHz @
30 degrees

7.1MHz @
15 degrees

14.2MHz @
10 degrees

14.2MHz @
5 degrees

21,3MHz @
5 degrees

28,4MHz@
5 degrees

 remark

quadloop20m 15h

2,24

-1,1

4,56

-0,58

3,26

6,74

top open

diamond20m 15h

2,55

-1,04

3,96

-1,26

2,19

6,76

top open

2elyagi 12h

 

 

7,24

2,03

 

7,7

(10m yagi)

2elyagi 15h

 

 

8,35

3,42

 

9,31

(10m yagi)

Inverted V 2x15m 10h

3,31

-1,62

0,54

-4,86

 

 

 

Pa8w vertical 12m+4 rad.

0,06

-1,67

-2,12

-5,82

 

 

 

Pa8w vertical+reflector

4,17

1,79

 

 

 

 

 

ground plane for 20m 12h

 

 

-1,19

-5,02

 

 

 

loop40m 11h

4,77

0,35

na

na

 

 

 

loop40m 11h side fed

-0,33

-1,7

na

na

 

 

 

cobwebb 20m 12h

2,37

-1,93

2,64

-2,58

 

 

 

cobwebb 20m 15h

3

-0,62

3,85

-1,11

 

 

 

cobwebb 40m 12h

3,38

-0,81

4,84

-0,32

 

 

 

cobwebb 40m 15h

3,88

0,35

6,66

1,76

 

 

 

The above antennas are all modelled, and some are built and tested.
Many, many more have been modelled but their data would clutter the table, as they don't really contribute anything.
quadloop 20m 12h means: a quad (square) loop, 20m wire circumference, 12meter top height over ground.
Note that all loops are put up vertically and bottom fed unless specified differently.
And all heights are top height, not boom height. (This is very important!)
7.1MHz @ 30 degrees means: gain dBi at 30 degrees take-off angle on 7.1 MHz.

Now look at the performance of the 20m loop on 14.2MHz: The blue row in the table shows the very nice figures for such a small antenna.
A full size two element Yagi at the same top height will yield only 4dB extra gain for DX radiation angles.
But the Yagi would need a sturdy, much heavier tower, a good rotator, and it would have a turning radius of at least 5.5 meter!

My loop uses a lightweight glassfiber push-up pole, one very lightweight glassfiber spreader of 4m20 long in the top, an even lighter spreader in the bottom,
and no boom at all. I put it up in 2 minutes, and down within 1 minute...

Apart from the sheer impossibility to rotate any yagi in my small garden, I think the effort will not pay off compared to the loop.
In fact, the loop can be put up much higher than the yagi, using a much lighter construction, and compensate the lesser performance by height.

This loop fits my wishlist best:

* Reasonable gain at lower radiation angles
* Some directivity (mainly for noise cancelling)
* Low profile
* Light construction
* Multi band operation (using a balanced tuner)
* Small turning radius.

Of course I use a rather short symmetric feedline and a symmetric ATU, so I am only interested in radiation patterns, not SWR!

Take a look at the following modelling output using Mmana, modelled at 15m top height.
At all bands from 40 to 10m the loop yields very useful patterns. 
Only at the 6m band, there's a lot of waisted radiation in the higher lobes.
The figs on the left show the current distribution in the loop. 

7,1MHz
14,2MHz
21,3MHz
28,4MHz
Top element opened
(Bi-Square)
51MHz
Top element opened
Conclusions:

As my experiments and simulations show, the performance of antennas may not entirely meet expectations or computersimulations in all cases.
Especially for a small lot, it is impossible to include all parameters of the antenna environment into the equasion, since the house plumbing and wiring, steel rebar, etc. will all couple to the antenna to some degree, and therefore distort the outcome. And there's also the local QRM environment; for example, verticals in my location show dramatic noise problems on reception. So, antenna modelling is great, but the results still have to be tested in real life.

Right now, the bottom fed loop is my antenna of choice for 20m and up.
This antenna has a small profile, small turning radius of 2.1m, and can be built very lightweight, since only one light spreader is necessary in the top, and the rest of the antenna stays in place using an even lighter bottom spreader, a few cable ties and gravity...
This also means that I can easily give it a lot extra height compared to a beam (which I can't put up here anyway) using much lighter hardware.
The performance of the loop seems to be even better than expected; in real life measurements it greaty outperforms any vertical I tried, much better than predicted by modelling.

So right now I am looking for a good method to automatically open the top of this loop on 10m, so I don't have to do that manually. The most promising method seems to be a 10m trap, but modelling shows it may cost about 2 dB on the 15m band, which is not desirable of course. For all other bands loss seems to be less than 0,5dB which is acceptable.

What I will do is build the trap concept, and bypass it with a relay, so I can measure exactly if real live loss is in accordance with the modelling data.

73, Wil.

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