HF TRANSMIT - RECEIVE ANTENNAS
EIGHT ESSEQUE UPDATE G8SEQ QTHR
Having made my 80 m “end fed” dipole for the
home QTH, I decided to experiment with portable antennas. I had never
liked the impedance matching transformer, as it is a potential loss and
mechanically weak. I decided to see if a deliberate mis-match would produce
acceptable results. I then realized that it would be possible to adjust
the match by adding & removing turns on the choke, effectively rolling
the coil up and down the feed line. This effectively moves the feed-point
of the antenna so that it now becomes an asymmetrical dipole. Then there
was a bonus the antenna produced less than 1.5 : 1 VSWR on all bands 160
m to 10 m except 40 m! (What is happening on 160 m, I’m not sure.
I’ve only had a local contact and a station heard me in OZ, but
I wasn’t strong enough for him to make out my call. Morse probably
would have worked OK but it was a Phone Contest!)
I then made an antenna for use on holiday in Spain. I just grabbed two
random lengths of E2003 and RG58, with BNC connector already fitted and
stuffed them in my bag. On arrival I measured up the lengths to see what
I could make & fit into the area available. It turned out that I had
18 m of E2003 and 4m of RG58. I joined these together first and made a
choke coil out of most of the RG58, about 20 cm diameter.
I then measured a quarter wave at 7 MHz ( 10.5 m) from the far end and
cut the co-ax and rejoined the inner to outer to make the feed-point (see
SPRAT 110 P35 ). There were no trees available near the apartment, but
luckily a stand of bamboo was growing nearby. Three 18 ft canes later,
the antenna was bent & shoe-horned into the small garden of the apartment.
(Fortunately we were staying with the owner who is an old friend of mine
and the only comment from the neighbours was to do with the height of
the new washing line!) After a few minor adjustments the VSWR was 1.2
: 1 on 7.050 MHz and less than 1.6 :1 on all higher bands up to 10 m EXCEPT
for 30 m where it was still under 2:1! So who needs an ATU? Theoretically
you should be able to fiddle around all day with the coil and get 1:1
on any band you choose.
PS I dumped the ferrite tubes on the original a long time ago –
they were never necessary!
(Click on the image to the right to see the full article!) |
Since
this iteration I have built and tested two other versions. Another 40
m version using E2003 throughout and a 20m version using RG174 and PVC
covered wire for the outer quarter wave. With careful adjustment, a 1:1
VSWR can be obtained on a single band. However a compromise setting can
make the antenna work on most bands with a VSWR of less than 1.5:1, with
the added bonus that the antenna loads up on the sub-harmonic as a physically
short antenna. ie one cut for 40 m will load on 80 m; a 20m one will load
on 40m ( I've no idea what the radiation pattern/efficiency is like in
the sub-harmonic, as I have only tried my 80m on top band with Bob G4GEE,
who lives only 500m away.)
Best Dx is PY on 20m, using the 80m version from home on 5W PSK31. I had phone contact with St Petersburg, 5W SSB, rcvd 59, sent 59 + 60 dB ( he was QRO at 100W) operating "portable" from back garden, using 20m version as sloper with high end at 25 ft and coil hooked around picnic table for tension.
Updated Version
Other End Fed Antennas
The following article, also reproduced from the same edition of SPRAT shows that great minds think alike. Andrew uses a very clever technique for impedance matching, but I think this is fussy to implement and restricts the antenna to one band and needs a specific type of co-ax (UR76) for his values to work. The updated version of the ESSEQUE antenna just has one coil to adjust and can be made of virtually any co-ax & wire.
Page last updated 23rd August 2008 73 de John G8SEQ