Other SOTA sites: SOTAwatch | SOTA Home | Database | Video | Photos | Shop | Mapping | FAQs | Facebook | Contact SOTA

Short antennas


#1

While many people are experimenting with larger antennas, I wanted something very small and fast to deploy. Thus I spent a while today making a vertical for 40m that telescopes down to just 40cm. It is only 3m long when extended and I was rather dubious about how well it would perform. A quick CQ on 7.032 got an immediate reply from Nik HB9EAA who was followed by a DL station. Reports were good - especially considering the size of the atenna. I will post some pictures if the WX is fine tmw.

The idea is that this antenna can be used on those awkward summits where there is not enough room or too many people to make normal antennas useful. What do others use in such situations?

73

Richard
G3CWI


#2

In reply to G3CWI:

Hi Richard,
I have also adapted my normal telescopic mast which supports the dipole.
Basically, added a half round galvanised spike to bottom of mast giving me
a decent earth (and ground support). Wound helical coil on bottom section of mast, with suitable taps, all taped into place. Length of wire, roughly qtr wave on 14mhz, is attached to mast tip, terminates in croc clip and, when pole is erected, can be attached to any of the tapping points on base coil. I even use the same coax lead as supplied for the SOTA beam - inner clip to coil, outer to earth spike! This gives me 14mhz straight, plus 10, 7 and, even 5 mhz with max coil, and no atu - obviously compromise on dipole performance but it does work! A counterpoise, in addition to the earth spike, would also help if space allowed. Hopefully, it will be in use on SP002 for Fun Day!

73s de Peter G3TJE+


#3

In reply to G3CWI:

making a vertical for 40m that telescopes down to just 40cm. It is
only 3m long when extended and I was rather dubious about how well it

Yes but you sure were using quite long counterpoise wire laying on the ground? That is the normal trick with the so called small antennas. The other possibility is to use high Q tuned loop antenna, where the losses are made negligible compared to the small radiation resistance. If your structure is very close to ground, the ground plane will become part of the antenna system. Plus you are often operating on a slope or next to a raide. These will change the radiation pattern quite a lot from the traditional dipole-up-20m-models.

73, Jaakko OH7BF/F5VGL


#4

In reply to G3CWI:

Thanks to everyone for the reports from the Cloud today. It was a successful experiment with QSOs all round Eu - most 2 way QRP. Some work remains but the concept works. I have published some pictures at http://www.flickr.com/groups/sota

The resonant frequency needs a bit of tweaking and I also need to decide if it is better to have the feeder coupled into the system as an additional radial or whether to isolate it with an RF choke.

Power used was 3W from KX1.

73

Richard


#5

In reply to G3TJE:

Wound helical coil on bottom
section of mast, with suitable taps, all taped into place. Length of
wire, roughly qtr wave on 14mhz.
This gives me 14mhz straight, plus 10, 7 and, even 5 mhz with max coil, and
no atu.

Hi Peter,

I’d be grateful if you could let me know the winding details of your tapped base loading coil. I have a 5m telescopic mast which I was thinking of using to support a quarter wave wire vertical for 14MHz, but it would be great if I could also use it on the lower bands.

Thanks, 73,
Fred, GI4MWA


#6

In reply to G3CWI:

As a result of yesterday’s tests on the Cloud I modified the matching system for the RSA to include more adustment taps. Conditions were breezy today on the hill but the system performed well with 18 contacts in 20 minutes on 40m CW. The first one was an s2s with Fritz DL4FDM/p so the RSA must be working reasonably well. I appreciated the small size and light weight on the walk today which was rather longer than normal as I walked from Buxton to Rainow (near Macclesfield).

Thanks for all the reports - much appreciated! My initial thoughts are that such a small antenna would be nearly useless on 40m SSB but it is perfectly viable for CW. I suspect that non-SOTA people would find such an antenna less satisfactory - the pool of keen chasers must be worth at least an extra 10dB!

73

Richard
G3CWI


#7

In reply to G3CWI:

Hi Richard,

Sounds like your antenna is working well. The Low Band DXing book by ON4UN has one chapter for these short verticals for fixed stations working on 80 m and 160 m bands. Of course the design philosophy is quite different for a fixed station for DX hunting compared to the portable light weight SOTA operation.

My idea was that for the weight you can choose to put the wire into a loading coil, use it for capacitance loading, lay it on the ground for the radials/counterpoise or hang it up for the antenna “radiator” (actually the counterpoise or second half of the dipole is also a radiator). I did not do careful case study however which of these produces the lowest weight with reasonable antenna gain.

73, Jaakko OH7BF/F5VGL


#8

In reply to F5VGL:

Hi Jaakko

I tune my antenna against two 5m long radials (and the outer of the coax as this is not isolated from the system). This seems to work quite well (the results are rather better than expected). The loading coil is wound with 1.5mm enamelled Cu wire with the last part having taps every other turn. A capacitor of 270pF + 68pF in parallel is across the coax. This forms an L network with the end of the loading coil to transform the low impedance to 50 Ohms (the value was determined by using a varable capacitor for testing and them measuring the required value on a capacitance bridge. The match is good but obviously quite narrow bandwidth as the Q is fairly high.

It would probably make a good mountaineering antenna although a longwire held up on ski poles wuld be even lighter. The vertical takes up very little room when in use - an advantage on some summits. It is also faster to set up than a longwire.

Sadly the type of telescopic pole that I use is not readily available.

73

Richard
G3CWI


#9

In reply to F5VGL:

Hi Jaakko

I tune my antenna against two 5m long radials (and the outer of the coax as this is not isolated from the system). This seems to work quite well (the results are rather better than expected). The loading coil is wound with 1.5mm enamelled Cu wire with the last part having taps every other turn. A capacitor of 270pF + 68pF in parallel is across the coax. This forms an L network with the end of the loading coil to transform the low impedance to 50 Ohms (the value was determined by using a variable capacitor for testing and them measuring the required value on a capacitance bridge). The match is good but obviously quite narrow bandwidth as the Q is fairly high.

It would probably make a good mountaineering antenna although a longwire held up on ski poles would be even lighter. The vertical takes up very little room when in use - an advantage on some summits. It is also faster to set up than a longwire.

Sadly the type of telescopic pole that I use is not readily available.

73

Richard
G3CWI