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HB9ABX > TECHNIK 06.06.20 10:35l 85 Lines 4081 Bytes #999 (999) @ WW
BID : 66UDB0FHN033
Read: GUEST
Subj: Evaluation od HF Antennas
Path: IW8PGT<IZ3LSV<DB0ERF<DB0RES<DB0OVN<DB0GOS<DB0RBS<OE5XBL<OE2XZR<DB0PM<
DB0FHN
Sent: 200606/0917z @:DB0FHN.#BAY.DEU.EU [JN59NK Nuernberg] obcm1.07b12 LT:999
From: HB9ABX @ DB0FHN.#BAY.DEU.EU (Felix)
To: TECHNIK @ WW
Reply-To: HB9ABX @ HB9EAS.CHE.EU
X-Info: Sent with login password
Evaluation of HF Antennas
The proper evaluation of the efficiency of HF Antennas is a very difficult and
time consuming process, if it is done seriously.
Listing of the number of contacts, signal reports, list of worked countries
etc. says nothing about the efficiency of an antenna.
In this way many publications describe the quality of new antennas.
With a piece of wire, connected to an antenna tuner one can perform worldwide
contacts under good conditions, while the best antenna does not allow to reach
any contact, if there are no propagation conditions.
Also, the SWR of an antenna does not say anything about the efficiency of an
antenna. The SWR only tells us, how good the transmitter is matched to the
antenna, but does not tell how good the waves are radiated by the antenna.
Based on the SWR, a Dummy Load would be the best antenna, as it shows an SWR of
1.0 on all frequencies ...
The problem arises from the very different HF wave propagation, which shows
strong variations dayly, hourly, and every minute.
A signal report of S5 may be good, while a report of S9+10 may be a bad report,
as it depends strongly under what propagation conditions the report is
obtained.
How to evaluate correctly ?
In order to evaluate correctly an HF antenna, this evaluation has to be done in
contacts with remote Stations.
The contact to a Remote Station is through sky waves, while the waves are
reflected by the ionosphere. In sky waves the antenna polarisation is no longer
important, as the polarisation changes permanently in a random, accidental
way.
To make a valid comparison, we need a defined second antenna with known
characteristics. Both antennas have to be tuned for lowest SWR and have to be
fed with equal power.
This is accomplished best by connecting both antennas through a coax switch
(A/B switch) to the same transceiver.
In order to prevent mutual coupling of the antennas, they have to be separed at
least by half wave length distance, and the coax cables have to be decoupled
by using suitable decoupling (current baluns).
Measuring in the near field, or in the ground wave is not suitable, as remote
contacts work through the sky waves.
What for serves a strong field on the ground, if hardly something is radiated
into the sky ?
During the contact with a Remote Station, multiple changeover between the Test
Antenna and the Comparing Antenna has to be done, to allow the Remote Station
to compare the signal levels of the two antennas, and to perform an averaging
of the signals in the QSB.
Comparation between two antennas!
At distance at long distance (300 km to 10'000 km)
- At the same time !
This has to be done with a high number of QSOs, as there are always some
"strange reports", as some operators are unable to read correctly the signal
level, or even confuse the call signs.
Furthermore, one has to make sure, that a real signal report is obtained, as
many stations just report 59 without observing the signal.
Contests are absolutely unsuitable for this purpose!
After performing more than 20 such correct test QSOs, one obtains the real
picture of the efficiency of an antenna.
This procedure requires a real big effort, but it seems to me to be the only
valid method to evaluate the efficiency of an HF Antenna.
Note: In case of directional antennas, the test antenna, as well as the
comparing antenna have to be directed towards the Remote Station.
And:
In this procedure it's absolutely insignificant what type of receiver is used,
and whether the receiver is calibrated or not. The only criteria to be observed
is the signal strenth difference between the two antennas, with the
determination which antenna produces the stronger signal, or if both antennas
produce the same signal strength.
As result one knows, whether the Test Antenna is better, worse, or equal to the
Comparing Antenna.
The absolute signal strength is totally irrelevant.
73 de Felix, HB9ABX, (felix-abx @ gmx.ch)
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