PA8W Amateur Radio

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

 

The 2x15m doublet: all about keeping losses down.  

The 20m loop: a simple multi band antenna with excellent properties.

My 17m endfed portable (DL/) antenna and its performance on 80 and 40m.

Beware of feedsystem loss!


In these pages I discuss the possibilities of simple but effective antennas for the HF bands, without ending up with crank-up towers and 5-element beams. Lots of Hams -myself included- simply lack the space for big systems, so they have to setlle with less.
Luckily, the amount of radiation from a transmitting antenna is not determined by antenna size. 
Instead, it is determined by antenna efficiŽncy, in other words, how well losses are reduced in the antenna system itself and in its vicinity (mainly ground!). So it is absolutely possible to build a small but effective antenna system.

However, I found that there's a lot of humbug in HAM-land, on the internet and even in some books. Though probably stated with the best possible intentions, it seems that a lot of "knowledge" is simply copied data from some untraceable source, without being tested or verified.
A lot of "wishdom" has settled as "known facts" in the HAM society, and in many cases this obscures the real understanding of what is going on.
I try to avoid making the same mistakes by using multiple measurements and proven methods such as modelling and I compare the outcome of the different methods to see if the outcome reproduces and therefore can be taken as a fact.
But for sure I will have made -and will continue to make- my own mistakes...

Considerations:

Ok, well, our first concern is the desired radiation pattern; which antenna will fit your lot and at the same time give you the best radiation pattern for your purposes. 
About the last priority should be a nice 50 ohm impedance: this is one of the least signficant properties of the antenna, since there's almost always a good method to get it right. 
We look into that further on a next page.

But for now, let's focus on performance of compact designs.

 


In my small lot this very compact 20m circumference loop
has proven to be a very effective antenna on quite a number 
of amateur bands.

My shack is right under the roof you see on this picture.
In fact, the telescopic antenna pole is only one meter beside my rig.

So, feed line length is very limited which is good for low loss.

There's a door right below the bottom of this picture, through which I can 
easily step outside onto the carport roof and do some antenna work or turn the antenna into another direction.

On the left you can just see the 2,5mm2 wire of my doublet dangling down, since I've  lowered it to make space for the loop.

To achieve good coverage of all amateur bands from 160 up to 6 meters, my experiments -and theory as well- show that I need at least two antennas.
Actually, I've got three, because it is very important to compare your antenna to some kind of reference antenna.
Quickly switching between the two gives a pretty good idea about performance.

You need more than one radiator for optimum coverage of the HF bands because:
1, If you force a big radiator to work on higher frequencies it's radiation pattern will break up in multiple lobes and notches, which is undesirable.
2, If you force a small radiator to work on low frequencies it will present a difficult match, which is very difficult to feed with low loss.
A radiator of whatever kind can be very efficiŽnt over at least one octave.
So a doublet cut between the 80m and 40m band can be an excellent antenna on both bands.
And a loop for 20m can still be very effective on 10m.
Both antennas can be used on bands beyond that but then the pattern gets disrupted (frequency too high) or feeding efficiŽncy will deteriorate (frequency too low).

So this is what I generally use:

1, A 2x15m inverted V doublet out of 2,5mm2 insulated copper, with ladder line feeder of the same wire strength.
Feedpoint is around 12m high.
This antenna excells at the 80 an 40m band for short haul (NVIS) distances, up to around 1500/2000km.

2, A 20m circumference vertical loop, made of 0,75mm2 insulated copper, horizontally polarized, with 300 ohm feeder. Top 14m high.
This one excells on the higher bands for medium to long distances.

3, A vertical, with a few bured radials, radiator 1,5mm2 insulated copper adaptable length up to 12m long, with 300 ohm feeder. 


Overall performance can be described as follows:

160m: reasonable performance of the doublet, no experience with the verical yet.

80m: very good performance of the doublet for distances up to 1500km. Beyond that distance, the vertical wins.
The vertical however is about 6dB more noisy on receive.

40m: very good performance of the doublet, loop almost just as good, absolutely better on receive.
The vertical is sometimes up to 6dB louder, not only on real DX, but always at least 6dB noisier on receive.

20m: reasonable performance of the doublet but the loop always wins hands-down, up to 12dB difference.
Never ever heard a signal stronger on the vertical, though the vertical is around 6dB noisier on receive.

17m,15m,10m: Same story but even more pronounced.


The above overall judgement has been formed and avaraged over the years since spring 2009, in very different band conditions.
My QTH is in a very flat area with pretty good ground conductivity (works out well mostly for the vertical mainly)
Lots of surrounding buildings but most of them around 9m high max.
High noise level due to computers, flat screen TV's etc.

What really sticks out is the performance of the loop on the higher bands. 
This really exceeds the data out of my modelling attempts.
Also, the vertical surprises sometimes on 40m, but quite unpredictable.
I am still looking for proper explanations...

So let's look at the modelling details; the following table lists some antennas I modelled, not all of them are actually built.

Many, many more have been modelled but their data would only 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.

 

 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

diamondloop20m 15h

2,55

-1,04

3,96

-1,26

2,19

6,76

top open

quadloop20m 10h

1

-3,59

-0,41

-5,93

-0,97

2,63

top open

2elyagi 12h

 

 

7,24

2,03

 

7,7

(10m yagi)

2elyagi 15h

 

 

8,35

3,42

 

9,31

(10m yagi)

Pa8w 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

 

 

 

The table shows that on 40m the inverted V (first grey row) is a pretty good antenna. On 80m (not listed in the table) its main radiation angle is 90 degrees so a very good NVIS performance is guaranteed as long as losses are kept low.

Obvious is also that the Vertical (second grey row) only becomes competitive on 40m for the lower radiation angles (DX). This however is not exactly the case in real life. I often notice a stronger signal on the vertical for signals from much smaller distances, like 800km or so.
This emphasizes the importance of real life test; they do not always live up to theory...

Now look at the performance of the 20m quadloop on 14.2MHz: Its blue row shows very nice figures for such a small antenna.
And in real life it seems to be even better still!
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 lot, 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.
And hight is an crucial parameter for horizontal antennas for DX angles.

73, Wil.


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