TRANSMIT - RECEIVE ANTENNAS
This file contains details of my antenna designs for HF/VHF/UHF.
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 6th October 2006. 73 de John G8SEQ
