INTRODUCTION
Loss in the matching Network and Coax Choke
With a capacitive reactance of around 340 ohms and a dissipation factor of 0.0094, the loss in each of the capacitors is around 3 ohms.
(3)Combined matching loss:- Considering each half of the network as a
loaded tuned circuit and adding the losses of the capacitor and inductor, the
total loss in each half is not more than 10 ohms and the unloaded Q = 340/10 =
34.
Each network half sees half of the assumed 2368 ohm load which equals
1184 ohms. The loaded Q of each half is then close to 1184/340 = 3.5.
Percentage loss equals (loaded Q)/(unloaded Q) x 100 = 3.5/34 x 100 = 10
% loss.
So each each half network looses 10% of the power fed into it
and as it shares the power with the other half, the efficiency of the whole LC
network is 10%.
(4) Possible loss in the coax isolation choke:- The unloaded Q of the
choke I have used is in the region of 100. As its reactance is over 1000 ohms,
we could assume loss resistance of around 10 ohms. The current flowing (if any)
either side if the choke is too small to measure on the coupled meter I have
used to check. It is certainly very much below 0.1 amp, but lets take the case
of 0.1 amp which dissipated in the 10 ohms would produce a loss of 1 watt.
Hardly a significant proportion of the 25 watts used in the tests.
(In
later tests, the choke was connected as a trap to increase the rejection
impedance. In this the loss was derived as 4%. For more detail, Click
Here).
From my figures, overall loss in the matching and choke circuit could well
be contained within 15% of the power consumed. Observation of Field
Strength variation with change in orientation of Antenna More Distant
Tests Further Tests
I later repeated the field stength tests at the 3.5 metres distance with the
trap down 1.5 metres from the input connector and a reversal of results
occurred. The field measured at right angles to the antenna was now around 6 dB
above that measured from the end of the antenna tube with the antenna
horizontal.
The results from the two tests seemed to inidicate that you might be able to
control the angle at which best radiation occurs by controlling the position
down the coax where the trap is placed.
In my next test I set up the antenna in a vertical position to measure the
different field strengths for the two locations of the trap. The field strength
measured was 11 dB higher when the trap was at 1.5 metres down the cable than
that measured when the trap was right at the input connector.
From these tests, it would seem that for low angle radiation, the trap is
better placed at 1.5 metres down from coax than at the antenna terminal
connector. I checked this out further with a test with my friend at the 11 km
distance. My friend reported that the EH was one half an S point below the
inverted V and this could be interpreted as 3 dB. With such a small figure in
hand, I should also mention that I measured the coax cable loss to the EH and
this was 1.25 db. I ignore the feeder loss to the inverted V as it is fed with
open wire matched line with negligible loss. So we can really subtract that 1.25
dB from the 3 dB to give a difference between the inverted V and the EH antenna
with trap at 1.5 metres, as less than 2 dB.
Some doubt had been expressed as to the suitability of using the inverted V
as a reference antenna. It was thought that the performance could be better
assessed by using a vertical reference antenna whose radiation characteristics
were more easily defined, particularly at a low angle.
Vertical Antenna Details
A 2.5 metre high vertical antenna was erected above a garage/workshop
building with its base level with the galvanised steel decking roof and using
the decking as a ground plane. The decking is 2.5 metres above ground level. The
antenna is matched to a 50 ohm coax cable with a Z match tuner coupled via an
SWR meter, both mounted at the roof decking level.
The antenna is around 1/8th wavelength and based on a formula I used, I
expect its radiation resistance to be near 5.4 ohms. The Z Match is a compact
coil version of the two coil Z match which I have referred to on my web site.
Measurements on the Z Match tuner indicated a 3 dB loss when loaded into an
equivalent of the 5.4 ohms load. The coax lead from the transmitter inside the
building is only a few metres long .
The 20 metre EH antenna with the trap at 1.5 metres down the cable is mounted
some distance away in a tree at a height of 2.5 metres above the ground. The
coax lead, fairly long, has a measured insertion loss of 1.25 dB.
Results of Comparative Tests
Tests were conducted with the same friend as I had tested with before at 11
km distant. I live partly up the slopes leading up to the Adelaide Hills and my
friend is on the flats. Communication could be considered as close to line of
site.
Using the same 25 watts power on each antenna with continuous carrier, he
gave the following reports:
The EH antenna was 0.3 of an S point
below the Inverted V. (On a previous report he had given 0.5 S point).
The EH antenna was 0.2 of an S point
above the Vertical antenna.
Making corections for the comparisons, the vertical antenna is down by 3dB
because of loss in the Z Match tuner. The EH antenna is down by 1.25 dB because
of loss in the feeder cable and down by 1 dB because of 20% loss in its matching
network, making a total of 2.25 dB loss. However its signal report was 0.2 of an
S point up on the vertical antenna which could be considered as 1 dB higher. On
these figures we could say the effective signal levels from the vertical antenna
and the EH antenna were almost the same.
Considering the inverted V to have negligible matching loss and the fact that
it was 0.3 S point (2 dB) up on the EH antenna we could also say that the
readings from the three antennas were very close.
I also noted the receive levels from his single sideband speech transmission:
I recorded the Inverted V as one S point
above the EH Antenna.
I recorded the Vertical antenna as one S
point below the EH antenna. Some
Conclusions Losses in the matching network and the isolation choke or trap are
contained within 15%. If the remainder of the power is not lost here, it must
surely be radiated in some direction or other.
With the trap or choke fitted right at the antenna input connector, much of
the radiated field is directed upwards. The field tends to be concentrated at a
lower angle if the trap or choke is fitted 1.5 metres down the cable from the
antenna.
With the trap at the 1.5 metre position, the signal received at distance from
the EH antenna compared very well with that from a full wave inverted V antenna
at the same location and fed with the same power. The EH antenna performed even
better when compared to the newer Vertical antenna.
There are those who insist that the EH antenna just works as a simple
shortened dipole. As such, with dipole elements just 0.5 metre long, it would
have a radiation resistance of a fraction of an ohm. To match this from 50 ohms,
most of the power would be lost on the inherent much larger loss resistance of
the matching network.
The question must be raised that if this is so, how is it radiating as well
as vertical antenna with a radiation resistance of 5.4 ohms?.
No longer can there be a claim that it is radiation from the coax - Current
down the coax shield has been blocked with a trap.
We are still
looking for a attenuation reported in some no coax tests. There seems to be
various verifications that the EH antenna radiates much like a vertical antenna
but it did occur to me that perhaps most of the tests carried out were with the
presence of the high coax shield current. If the EH antenna operates in
isolation without the coax radiation, just what field pattern results from the
two phase field set up? Could a lot of the radiated field be concentrated in
line with the dipole plates and not just at right angles to them? This led to
the following series of tests:
The EH antenna has a 11.5 uH coax choke
resonated as a trap with a parallel capacitor, fitted at the input connector
feeding the matching network. The antenna matches up beautifully and there is
absolutely no interaction with the coax line which can be moved or altered in
length with no change to the antenna. Testing for the presence of any
coax shield current was carried out. There did not appear to be any
radiation other than that from the vicinity of the antenna and its matching
components.
The antenna was mounted near 2.5 metres above the ground. A
field strength indicator was mounted at a similar height above the ground and at
a distance of 3.5 metres from the antenna. This is about 0.175 of a wavelength
and is the furthest distance I could go to get sufficient reading on the passive
indicator for the tests with 25 watts fed to the antenna.
Readings for
different orientations are referred to those measured at right angles to the
plane of the antenna when it is mounted in the vertical position. (i.e. 0
dB)
The antenna was rotated to the horizontal position. The field
strength at right angles to the antenna plane was -2 dB.
The antenna,
still mounted in the horizontal position was turned so that the field strength
indicator looked towards the matching network end of the antenna tube. The field
strength was - 6 dB.
The antenna still in the horizontal position was
turned 180 degrees so that the field indicator now looked towards the dipole end
of the antenna tube. The field strength was +6 dB.
With antenna returned
to the vertical position, its bottom (at the matching network end) was then
pulled back to slope at 45 degrees and in a direction away from the field
indicator. The field strength was - 2 dB.
The 45 degree slope was then
reversed so that the antenna bottom was pulled forward in the direction of the
field indicator. This, in effect, monitored the field strength at an angle 45
degrees to the normal antenna vertical plane. The field strength was +6 dB.
Assuming for the EH antenna, in its own right, that 0.17 wavelength
monitoring was sufficient to be in the 1/r region and radiation was the
prominent field, the tests appear to indicate that the radiation field is 6 dB
higher off the top of the antenna tube and in a 90 degree cone above the
antenna, than at right angles to its vertical plane.
The test also seems to
indicate that the antenna is better for high radiation angles than for low angle
and this explains some of the apparent loss of received signal experienced in
short distance, line of sight, tests.
I later carried out
some air tests with a friend 11 km away. (Again with the coax current choked off
at the antenna input connector and no detectable coax current.).
His report
confirmed that the results of my tests close to the antenna were fairly valid.
He reported that the received signal was one S point higher when I transmitted
with the EH antenna top end facing him than when the antenna was vertical. The
same for when the antenna top was tilted forward towards him by 45 degrees.
On a previous occasion I had made a test with my friend to compare the
EH antenna in the vertical position with my full wave inverted. As before, the
signal was two S points below that of the inverted V. Off the top end of the
antenna or with it angled, the signal level was one S point below the inverted
V.
I think the vertical radiation may account for a lot of the signal
loss that has been observed in some tests when there has been no coax to provide
low angle radiation from its shield current. If one could utilise much of the
energy radiating upward from the vertically placed antenna so that it reinforced
that from at right angles to the antenna, perhaps one might see a result quite
comparable with the reference antenna.
I think the tests demonstrate that
this antenna, with dipole elements just 2.5% of a wavelength high, can put out
quite an effective signal without the help of coax current assisting with
radiation. When tilted forward, it put out a signal 11 kilometers distant at
merely one S point below that from the antenna of one wavelength long.
I again refer to the
tests with the trap fitted right at the input connector on the PVC tube. These
indicated that the field was around 6 dB higher off the top end of the tube than
at right angles to it.
