Portable antenna with gain

[northern35s]

So I've been thinking about gain, and as some of you may know I operate exclusively mobile, for those that didn't know, now you do
I also only use the amateur bands for SSB, although I still use the UK CB allocation and mode (FM), but no where near as much as I do amateur frequencies.

So that's the bit about me and what I do, now back to the subject matter, a portable antenna with gain. I wanted to build an antenna I could take to the beach / seaside and benefit from vertical polarisation and enhanced gain. Some of the ideas were phased verticals, I have a couple of friends who use portable phased verticals, and whilst only 1/4WL long (relatively short), they require a good set of radials and a phasing system. neither of this are difficult to make, but they do add to the complexity of a system that will be used in public and put up and taken down in the same session.

So this got me thinking, what about a Yagi antenna, not a horizontal, but a vertical Yagi, one that will be ground mounted and benefit from operating over, or near, salt water. The benefit of ground mounting is no need for a tall tower, simple construction, and if fed at the base of the driven element, no complex coax support or interaction from a drooping coax. Ok so how do we make it a driven element with a base feed? Ok so it's not truly fed at the base of the element, it uses the same principle of the coax dipole, the coax comes in at the base, but relies on common mode current to create the lower section of the antenna.

So I've overcome the feed arrangement, how do I stop the common mode current at the base, well I could use a coil of coax, but efficiency wise that is the worst choice, not a bad choice, but could be better. If I follow Steve Hunt's (G3TXQ) chart I can see that an FT240-61 torroid, with 8 turns of RG 58, will give me excellent choking impedance at my desired frequency (28.5MHz). If I enclose the torroid in a small project box I can also add SO239 connectors to the box for the antenna element and the feedline to connect to. This tidies things up again, simple and clutter free.

That's common mode dealt with, now how do I deal with the low feedpoint impedance, around 24ohm, I had to think about this one for a while, but hit on an idea, what does a 1/2WL element have at either end? very high impedance, and at the centre I know it has a low impedance, somewhere along either the upper or lower half of the driven element will be a point that matches to my coax impedance, all I have to do is find it. Well I've been using MANNA-GAL to model the antenna, so I just adjusted the feedpoint on the driven element and hey presto, 50ohm. So the antenna has an off centre feedpoint, and if I make the lower portion of the antenna shorter, and the upper portion longer, I also save on coax usage (ok so I'm tight with my money ).

So hopefully I'm building an image of what the antenna is, to use in the field it will require support, a couple of 6m fibreglass telescopic fishing poles comes to the rescue there, with connecting paracord between them I can maintain the 1m separation between driven and director elements, I also use four lengths of paracord to keep the poles upright, two on the driven pole and two on the director, with the connecting rope supporting each other.

Ok no pictures yet, I had made a video earlier on and realised I'd done it in portrait mode rather than landscape, and that just won't do, so I'll make another video tomorrow along with some pictures for your delectation.

What I can show is a model of the antenna over sea water, you can see the front to back, the elevation angle and gain:

Ignore if you will for a moment the higher than 50ohm feedpoint in the model, I made some changes to element diameter and feedpoint which changed things a little when I took the snapshot. I will also give the specific dimensions when the antenna is tuned and tested along with the final modeling.

Hopefully it's given you some ideas to try of your own, my intention is to make one for 20m (my favourite DX band of choice), but wanted to make a smaller version first, I can also make different elements covering other bands, once the 20m version is complete, as I already have the 10m long support poles and I'll be making another, more wideband, choke. The simplicity of using SO239/PL259 connectors as the antenna base

What you can also see in the model is the very wide beamwidth, this is perfect for my usual location that has a great take off to N, W, S, this covers Australia and Pacific long path, New Zealand short and long path, USA East, Mid and West short path, Caribbean and South/Central America short path.

Does it work? Well it isn't properly tested yet, however I used it on the UK CB band the other night and can confirm F/B was around 10-12dB with local stations, this was with the antenna resting against my wall and moving the director element in front, and then behind, the driven element, up to 2 S points reported by local CBers, I can't wait for Monday when I'll get a chance to use it by the beach :w00t:

 

[Marconi]

35, I don't have anything to add at the moment, but I encourage you to continue, and keep us posted on your progress with the real antennas compared to the models you make using MANNA-GAL. It would be good if we could see a model predict close to what we might experience in the real world.

I'm just too old to do the real world thing anymore.

Good luck,

 

[northern35s]

35, I don't have anything to add at the moment, but I encourage you to continue, and keep us posted on your progress with the real antennas compared to the models you make using MANNA-GAL. It would be good if we could see a model predict close to what we might experience in the real world.

I'm just too old to do the real world thing anymore.

Good luck,

I just hope my explanation made sense, pictures would have been so much easier, as would the video, but I messed up with the video and then got a call from my nephew to go and tear the dirt up with his motorbikes on some waste land, I couldn't turn the offer down

The downside was the argument I had with Mrs northern35s afterwards, when I told her I was going to buy my own dirt bike as I'd been using my nephew's for too long ---- the argument is set to continue until I get my way, a bit of DIY and some fashionable incentives her way should sweeten the deal :laugh:

 

[vkrules]

What I did was use wire v shaped parasitic elements ( sort of like a 3 element moxon) and used two 2 meter fishing poles as the "boom" that way the lot could be mounted on one 10 meter pole and could be turned . The t2lt works well as the driven element I used 16 turns on a 65 mm former( siros dimensions on the gain master)for my choke .A few feet of coax is cheaper and easier to find here than ferrite and you can slide the coil down over the mast to keep it tidy.Looking forward to hearing how you get on.

 

[northern35s]

What I did was use wire v shaped parasitic elements ( sort of like a 3 element moxon) and used two 2 meter fishing poles as the "boom" that way the lot could be mounted on one 10 meter pole and could be turned . The t2lt works well as the driven element I used 16 turns on a 65 mm former( siros dimensions on the gain master)for my choke .A few feet of coax is cheaper and easier to find here than ferrite and you can slide the coil down over the mast to keep it tidy.Looking forward to hearing how you get on.

The vertical dipole antenna is of similar similar construction, as used on the TX6G DXpedition recently. The decision behind my design is the simplicity of construction and feed arrangement.

I hear what you say about the choke, I bought a load of 43 and 61 mix cores at a radio rally a couple of years ago and have pressed them into service on a number of antennas, including my mobile antenna. Using this style of choke I can change bands very easily by slaving in a different element, unscrew one PL259 and screw the new one on

I can't wait to test it, but I have this and another night shift to get through first, however I will probably take some photos tomorrow before I head out to work.

I have plans for a multiband version too, but I've yet to model that, the downside being compromise if I want to keep the design simple, but that's all long term, I'll concentrate on the monobanders first.

 

[northern35s]

Here's a quick video with the antenna lying on the ground, I'll be carrying out some tests later today and will have pictures and video to show the results:

G7DIE portable 2 element vertical yagi

Remember gain figures are modelled over salt water as this is where the antenna will be used

 

[Marconi]

Here's a quick video with the antenna lying on the ground, I'll be carrying out some tests later today and will have pictures and video to show the results:

G7DIE portable 2 element vertical yagi

Remember gain figures are modelled over salt water as this is where the antenna will be used

Very good work 35. I always appreciate some evidence of products being tested, and seeing the signs of some work to do it.

Is there a calculator or the math for this idea on the Internet...that provides the dimensions? It looks technically difficult, but I would like to try and model it.

If so, would you give us the link you used?

I understand this is designed to be portable for your purpose, so is the radiator mounted at or near the ground?

How does this setup compare in performance...if not mounted over the sea water or nearby?

 

[hotrod]

for portable use on amature bands id try a a99. thier simple they will tune to
other bands w/ tuner. and they might offer a tiny bit of gain

 

[W5LZ]

You can make almost anything 'tune' to almost any band if you want to bad enough. The 'catch' to that is that doesn't say how well that thing will work on a band it's not at least close to what it was designed for. An 'A99' can certainly be made to work on bands not 'close' to it's design frequency (10 and 12 meters is close to the design freq so they aren't much of'a stretch). Any further away than 10 or 12 meters just isn't going to work very well.
For any antenna to work well it has to be at least close to resonance and impedance matched to the rest of hte system (SWR). Leave one of those qualities out and it's not going to be very good of an antenna. When using a tuner (or the 'tuner' built into an 'A99') you are only dealing with impedance matching and nothing to do with resonance. That means the transmitter may not object as far as the SWR goes, but that antenna isn't going to be as efficient as it should be. How efficient is that? No clue, but it ain't gonna be good.
Have fun...
- 'Doc

 

[vkrules]

Ok looking forward to seeing the next vid. Come on hotrod, northern is making a 2 element beam why would you even mention a a99, are you going to fit a parasitic element to it? His driven element alone would outperform the a99( if he changed the feedpoint back to centre fed). Actually fitting a reflector to a a99 might be a good project .

 

[northern35s]

Ok Chaps, I finally got chance to take the kit out to the seaside and have some fun, well a kind of fun, 10m was deadsville, but not beyond redemption.

One of the important criteria for this antenna was that it had to be capable of being assembled by one person, two people is always a bonus in such cases, but one person has to be able to do it. Almost as soon as I parked up I saw a friend pull into the car park, he came over and offered to give me a hand straight away, I told him it was essential I raised the antenna myself and that whilst his offer of assistance was nice, I didn't want it, politely of course. Here's the kit ready to be assembled:

Well putting it together was very straight forward, and erecting it went according to plan so no problems there. It still isn't finished as there are a couple of things that need tidying up, some quick attach plastic snaps for connecting guy ropes are yet to be sourced, but all in all it was easy. I did let my friend help to take it down though

Here's a couple of shots of the antenna erected, this one looking to South America:

This one with the DX wagon in the background:

Whilst you can see it in the pictures above, here's a closer look at the feedpoint/choke connection:

I've made a video, but you'll have to wait for that, it's 00:25 here and I haven't been to bed yet after the previous nightshift, so I'm a bit knackered to say the least.

The antenna performed well, if a little short for my desired frequency as the VSWR was a bit hot, I actually expected this as I was trimming the elements the other day when it started to rain and I cut around 6" off the main element that I shouldn't have, I'll add that and take my analyser out with me for the next test.
I mentioned earlier that 10m was dead, well not completely, it was a bit of a ghost town, but there were a few South American stations to be worked with a bit of band scanning. In all I made around 6 contacts into Brazil, Argentina, Venezuela, and Paraguay, so not a complete waste of time. I was also joined on frequency by a local friend who reported around 10dB difference between the side of my beam and my car mounted 1/4WL, given that the vertical Yagi maintains a very wide beamwidth then this is to be expected. The only real way to test the antenna is with a field strength meter, for this I'll need someone to help me locally as my equipment is a bit bulky, SDR receiver, tripod mounted antenna, attenuators, and a laptop, the chances of northern35s junior helping is none existent, he thinks I'm a nerd :headbang

Eddie, I'll get the details for the antenna design for you tomorrow, but for info I used a basic design in MANNA-GAL and edited the lengths for 10m, I then offset the feedpoint to match it to 50ohm, trial and error stylee

 

[Marconi]

Ok Chaps, I finally got chance to take the kit out to the seaside and have some fun, well a kind of fun, 10m was deadsville, but not beyond redemption.

One of the important criteria for this antenna was that it had to be capable of being assembled by one person, two people is always a bonus in such cases, but one person has to be able to do it. Almost as soon as I parked up I saw a friend pull into the car park, he came over and offered to give me a hand straight away, I told him it was essential I raised the antenna myself and that whilst his offer of assistance was nice, I didn't want it, politely of course. Here's the kit ready to be assembled:

Well putting it together was very straight forward, and erecting it went according to plan so no problems there. It still isn't finished as there are a couple of things that need tidying up, some quick attach plastic snaps for connecting guy ropes are yet to be sourced, but all in all it was easy. I did let my friend help to take it down though

Here's a couple of shots of the antenna erected, this one looking to South America:

This one with the DX wagon in the background:

Whilst you can see it in the pictures above, here's a closer look at the feedpoint/choke connection:

I've made a video, but you'll have to wait for that, it's 00:25 here and I haven't been to bed yet after the previous nightshift, so I'm a bit knackered to say the least.

The antenna performed well, if a little short for my desired frequency as the VSWR was a bit hot, I actually expected this as I was trimming the elements the other day when it started to rain and I cut around 6" off the main element that I shouldn't have, I'll add that and take my analyser out with me for the next test.

I mentioned earlier that 10m was dead, well not completely, it was a bit of a ghost town, but there were a few South American stations to be worked with a bit of band scanning. In all I made around 6 contacts into Brazil, Argentina, Venezuela, and Paraguay, so not a complete waste of time. I was also joined on frequency by a local friend who reported around 10dB difference between the side of my beam and my car mounted 1/4WL, given that the vertical Yagi maintains a very wide beamwidth then this is to be expected. The only real way to test the antenna is with a field strength meter, for this I'll need someone to help me locally as my equipment is a bit bulky, SDR receiver, tripod mounted antenna, attenuators, and a laptop, the chances of northern35s junior helping is none existent, he thinks I'm a nerd :headbang

Eddie, I'll get the details for the antenna design for you tomorrow, but for info I used a basic design in MANNA-GAL and edited the lengths for 10m, I then offset the feedpoint to match it to 50ohm, trial and error stylee

Well 35, it's not a big deal, but several of your links did not work in the original window of this thread for me. However, when I clicked on the button to "quote" you words in response...the new window shows all the images you posted. I'll try re-loading this thread again and see what happens.

Your setup looks a lot bigger when raised up on the beach, and several of my questions were answered with these views.

I was thinking that you may have used an Internet calculator for the dimensions for this particular antenna. Modeling this idea and then building the real antenna from the model's dimensions is great. This is the way models are supposed to be verified. We may get to see if modeling can give us a close prediction.

I don't know for sure if that can or will happen, but some folks on this forum are emphatic making the claims...it is not possible for modeling to predict performance accurately for several reasons.

 

[Marconi]

35, your images are fine.

 

[M0GVZ]

For any antenna to work well it has to be at least close to resonance and impedance matched to the rest of hte system (SWR).

Is the wrong answer. My inverted L on some bands had a modelled SWR of 85:1, was nowhere near resonance on those bands yet it had lobes with gain > 10dBi on those same bands. An antenna coupler allowed the radio to see 50 Ohms and put almost its full power into the radiating element and in fact for the antenna coupler to work properly it wanted a length that wasn't resonant on any band.

 

[northern35s]

This is the way models are supposed to be verified. We may get to see if modeling can give us a close prediction.

I don't know for sure if that can or will happen, but some folks on this forum are emphatic making the claims...it is not possible for modeling to predict performance accurately for several reasons.

I'll answer that by starting another thread so we don't derail thins one

How does antenna modelling help us