Hi Andrew,
I have seen it written that one loses 50% of the signal through the use of the stub to match the J-Pole to the rig. Even if that is so, my practical test of a 15 metre version of one of these J-Pole antennas on Tuesday this week go me 4 North American contacts on just 5 watts of SSB, with reports back being 5-5, 5-5, 5-9, 4-4 - of course the antenna at the other end would be doing a lot of the work, however that first report was from a chaser with an 80m Delta loop using it on 15m - so hardly a high gain beam!
With good radio conditions, everything is possible, with bad conditions, every bit of gain you can get is important.
a few comments on the losses in the stub. Your 50% or 3dB figure seems a bit high.
I think the problem with using a quarter wave coax line with a very high SWR is mainly the voltage rating of the cable. If the impedance at the top of the quarter wave stub is 2000 ohms, to match the impedance of the half wave end fed, the voltage is much higher than it would be for 50 ohms. eg 50 watts in an impedance of 2000 ohms would be 316 volts which is ok for quality RG58 but possibly getting a bit unfriendly for thinner coax. Arcing may occur at that power level in lower quality or thinner cable. At 5 watts, the voltage for 2000 ohms is about a third of that (sqrt(10)). However (obviously) the high voltage means treat it with caution, be careful with insulation and protect the connection physically. Whatever you do at a high voltage point on an antenna can detune it too.
The losses in coax are generally due to skin effect in the centre conductor, so cables with a thicker centre conductor have lower loss as the RF is spread around a lot more conductor. High current points in a coaxial line due to mismatch can cause higher losses than high voltage points, as long as the voltage is not so high as to cause a flashover. (no 1kw amplifiers on your rg58 stubs! 1kw would produce about 1.4kv and I think that would definitely be an unreliable antenna!)
I can confirm Ron was not thinking of me Andrew (VK1AD). Anyway to the subject of J-Pole EFHW antennas.
Yesterday from Goorooyarroo VK1/AC-036 (say it slowly) I worked S2S with Brad JS6TQS on JA6/ON-009 in Okinawa Prefecture. For the S2S QSO exchange my antennas were mono band J-Poles (EFHW) for 12m and 15m (QSOs were completed on 24.955 and 21.300 MHz respectively)
In addion to the mono band verticals I had set up a linked Inverted V dipole supported by a 7 metre pole. Having a two antenna arrangement, selected via an antenna switch provides the opportunity for the operator to form a balaced and objective view of the respective antennas performance over real ground, on a real mountain top.
Not only did I work Brad on the J-Pole antennas I also worked SOTA chaser John ZL1BYZ plus a range of chasers in VK.
I have used RG58 coax for construction of the J-Pole stub, I would not have considered using a narrow section coax such as RG-174, mainly due to its losses but also I like the option of the 857D to âcrank upâ the power output when required. Since constructing my first J-Pole, a 10m version late last year, I now have a collection of J-Pole verticals for 20, 15, 12 and 10m. Construction of a 17m version is underway. The success or otherwise of using this type of EFHW vertical antenna (not an inverted V) can be gauged from my SOTA S2S log.
To those folk who are sceptical about the performance or otherwise of the EFHW mounted vertical or as an inverted V, my challenge to you is to construct a real EFHW antenna, not a computer model, and try it out from a real summit rather than a computer simulated summit.
I have just come up from my workshop in the cellar, having constructed J-Pole antennas for 15, 17 & 20 metres, which I will test on the antenna analyser tomorrow all being well, with the weather.
My âin-use from summitâ exeriences with this design of antenna have been positive, with contacts into the USA, Canada and Spain (15m) on 5 watts and Greece (20m) with 25 watts.
These latest J-Pole antennas have been built with RG-174 purely to reduce size and weight. I accept that RG-174 will have greater losses compared to RG-58A (3.9 vs 1.4 dB/100ft at 10MHz), however as I use a maximum of 12 feet of coax (including the covered element and feed coax) - this is 0.468dB loss with RG-174 or 0.168dB with RG-58A (in both cases a minimal amount of signal loss).
As for power handling, RG-174 on HF âshouldâ be able to handle up to 350 watts where RG-58A handles 1000 watts. It certainly isnât going to be a problem with my 20-25 watts that I can run from a summit.
Maximum operating voltage (Non-UL) for RG-174 is 1100 volts compared with 1400 volts for RG-58A.
I have been told that the thinner diameter of the co-ax can cause issues with the lengths required even though the velocity factor of RG-58 and PVC RG-174 is the same at 0.66. Tomorrowâs tests will clarify how correct this belief is.
At 100 grams per antenna including their plastic bags makes them really useful,along with the fact that no radials / counterpoise is required.
At risk of repeating some of what this extensive thread has covered, hereâs details of my 40m center-loaded vertical. It is cut for a 7m squid pole but I use it on a longer one which places the feedpoint about a meter and a half above ground. I use either this or a 40m dipole for 40m activations in VK3.
Anecdotally, my experience over the last year with the two antennas confirms what others have plotted or stated. If 40m has NVIS propagation (not common these days), the dipole is better than the vertical, and I will work VK3s. If not, I do much better on the vertical into VK5, 1, 2 and 7 in the middle of the day, and further into VK4 and ZL at dusk.
As a receive antenna, the vertical is 1 to 2 S-points down on the dipole/vee.
The vertical is much quicker to put up than the dipole.
Iâm thinking of trying out the end-feed, center-fed vertical dipole as described at http://audiosystemsgroup.com/VerticalDipole.pdf . Seems easy enough to make and I have all the parts on hand, including the big type 31 clamp-on ferrite.
Looking to the group for some collective wisdom on how best to tune the two elements in this setup. Should the coax âsleeveâ portion be tuned without the second element connected at all, or better to fiddle both elements until it gives the best reading? How would you approach it? Velocity factor for the lower section is given in the article as about .97 , so the lengths should be pretty close. (I assume that VF is what it is because only the outer part of the coax is being âusedâ for the lower element and has insulation over it; itâs not seen as âcoaxâ in this instance, just a large diameter insulated wire)
My hope is that I can make this quickly adjustable for 3 bands (15.17,20) by moving the choke on the bottom and folding back the top wire. Calculated values donât always translate to the real world, so some kind of adjustment is anticipated to get close to optimal so those places can be marked.
I would expect the lower element to need to be a bit shorter, because it is closer to the ground. That will add some capacitive loading, which will make it electrically longer than its physical length.
Because this is a center fed dipole, you could build a fan dipole. Multiple parallel elements on the top and two extra wires on the bottom. Or parallel top elements and move the choke. Ladder line is pretty closely spaced, but might work for two of the elements.
I donât think the velocity factor matters for the bottom element. This may be that rare case where K9YC is wrong. The velocity factor affects the transmission line behavior and is determined by the internal geometry and dielectric. The lower element of the dipole uses the outside of the coax shield as a conductor (skin effect), so that is not affected by the inside of the coax (spacing and dielectric).
One more thing. I think the choke might act as a resistive termination for the lower element. That could make it less efficient. But Iâll have to think more about that.
Regarding lightweight vertical antennas, this is an interesting solution:
SOTA antenna by Igor OM3CUG: uses 4 - 5 meters long fishing pole, 2 x 10 m wire in âM configurationâ, multiband operation, no feedline, fed directly via L tuner and a 1:1 current balun (both homemade).
I am thinking about giving it a try next spring, and would like to know if anyone else has used this design so far.
Yes, but (according to mmana) seems to work as well as my attempt of 2 x 10 m inv. V SOTA doublet (apex on 8 m fishing rod, fed with 300 ohm twin lead), while being much lower, without feedline, with smaller footprint.
It has a feedline. The two wires going up form a tapered balanced feed. Itâs a common system for printed microwave antennas but the design you have shown is a novel use. If it works mechanically for you there is no real downside that I can see. Go for it!
For the last 2 years, I used a dual band vertical dipole (20m + 10m).
I think that a good vertical is the best antenna for summits which are close to seaside, like those I have around.
Working SSB + QRP (usually 1-2 watts), while chosing the right time (ex: evening or late night), it was possibile for me to have dxs with Nord America, Canada and Carribean Islands, and Japan.
During sunlight, only european and north-african countries were workable. In the middle afternoon, I often received India. Surely propagation didnât help that much and the personal experience made great difference.
I use to install my vertical on a grp mast, something like a strong fishing rod, about 12m long. Where I can, I install it on the edge of the peak, facing the depths below to obtain the best take off angle.
My second option is a classic full-size multiband dipole, only for the chance to cover multiple bands.
I am using the MFJ 1979 with a loading coil, a small tuner and a tripod since years for my SOTA and GMA activations and normally it outperforms my linked dipole (configures as an iverted-V using a 6m-pole) at the same location. I have tested both antennas and the Buddipole in the field at a locations with bad ground values, which I have measured (I am geologist and can use professional equipment for testing ground resistivity at the working frequency - not at DC ! - and also permittivity or dielectric constant). The results are straightforward, if you think about antenna specifications:
The loaded vertical is used in the field with only 1 +/- resonant elevated radial. The hight of the elevated radial is - if possible - in the range of 0,5 to 1 m. Lesser is possible, but degrades efficiency very quickly especially on poor grounds like on most of our higher mountain tops.
A verical like this shows the upper-and-outer configuration and can be deployed within minutes. For short and medium range distances it is inferior compared to a dipole and an inverted V on my 6 m pole, but for DX it has shown to be unbeatable though overall radiation efficiency is clearly less than that of a dipole. All DX I made from summits is done using this antenna. The reason for DX performance is the well known low radiation angle. For short to moderate distances reception is 1 to 3 S-units less than that of my inverted-V, but I never had problems in working German or Central European stations. The signal strength of these stations is so high, that there wonât be any problems. On the other hand DX with an inverted-V has been impossible becaus the signal strenght of these stations here is really low and the degradation of the regional signal coming under high angles is really appreciated by me.
I clearly prefer the dipole in 40 m and lower and will give it a try definitely on 60 m duringe one of my next activations. Especially on 40 m during daylight DX is impossible, low radiation is useless and steep radiation is needed for more or less local contacts.
If I am not using the MFJ whip I take a 6 m pole with me. It can be used to fix the inverted-V near the top and also to hoist up a loaded vertical made of wire. If I use it only for 20 m and higher there is no need for loading, but a small tuner (smallest available is ok for 5W only) is recommended. If the range of the tuner is sufficient, one can also match 30 m without coil, but in this case the feeding line between antenna and tuner should be as short as possible. Nothing degrades the efficiency of the whole antenna system more than long an mismatched feeding lines.
Earlier I mentioned the buddipole. I know that it is expensive and heavier, but I can easily carry it with me as long as there is no real climbing to be done to reach a summit. Iâm using it with the long whips. So efficiency for 20 m and above is not as bad as one might expect. It has 2 pros:
It is a verry âstrongâ and well designed antenna.If you fix it with 2 or 3 ropes to prevent falling down it will withstand neary all weather conditions under which SOTA activities are thinkable. Last year I took it up on the summits of our Hercynian Mountains and it worked under snowfall and so heavy winds, that I wouldnât even have thougt about any other antenna. I never had any difficulties ist the configuration of the antenna under whatsoever conditons weather and temperature conditions.
It can be used from 40 m to 2 m with a lot of configurations in a very flexible way. The possibility to use it as a true vertical dipole - which I never tried on a summit, but at other portable locations - is a real pro for DX.
I think the most preferable way for a satisfiying activation is a short and lightweight (6 - 8 m) pole with itâs possibility to use it as a vertical with elevated radials as well as the usage as a dipole. The additional weight for a second set of wires is negliable and the possibilities for local and DX traffic are somewhat maximized. At QRP-levels the most important thing is not to loose to much efficiency in matching feeding.
Nice analysis Paul. I also made a center-loaded vertical for 40, but itâs my home antenna, not ever used for SOTA. It began life as a loaded drooping dipole with 8-meter long legs, mounted about 15 meters. After a year or so of fairly good DX results I spun it 90 degrees and ran the vertical part up a wood and bamboo combination, and stretched the other leg out to the north. I had read that one raised radial is both efficient and directional. I experimented with an opposite-facing (full-1/4wl) radial that I could switch to remotely, and indeed there was some directional effect. Eventually I added 2 short radials and it is a very good DX and near-DX antenna. I now plan to take it down and make a full 1/4 wl VGP with equal radials, or perhaps (after I recheck published data) the vertical element will be about .27 wl and the radials about 0.2 wl. 73 Steve in Okinawa
And this is my first post to the Reflector.
Iâve been a portable type of operator for the last 3 years after having a relatively large station prior to moving.
Iâm new to SOTA activationâs as well. I have in the past used an Alpha military Ez portable antenna. Those not sure of what it is, its a 13â tent pole and a 5:1 UNUN. Set-up time can be quick but performance can be lacking.
I recently purchased the MFJ 1979 whip to try and make things faster with better performance. The post from DL1AIW has got me thinking about streamlining my counterpoises back to 1 to make it faster, thank you. I shall try that and see if it works.
Further tests comparing my tri-bander J-pole against a commercial (LambdaHalbe) one gave some unfortunate results - my home made antenna was down 3 S-points on both receive and transmit compared to the commercial single band one. And while the commercial antenna costs between âŹ26 and âŹ36 depending upon band, it really isnât worth building my own (as the parts cost me that anyway!).
