NVIS 40m Dipole Mobile Whip Antenna

Mmm...

Dipole S)-239 to 3/8 dipole whip adapter arrived...




Here's a video (not sure this will work)

40m Dipole Whip Video

This is the option I've gone for to provide an NVIS Propagation to  

GB7CIP CATERHAM ON THE HILL   NGR: TQ33485719 [IO91WH]

Packet BBS in London from my QTH in Shrewsbury, absolutely no idea whether it will work, but maybe most of the daytime with usable F0F2 Critical Frequency, fingers crossed, using no more than 10 Watts SSB.

Wish me luck using a 16ft Dipole on a 20ft Lattice Tower...

How sturdy is the centrepiece? How are you going to mount it?

I've looked at them before but can't decide if it's going to be strong enough.

The MFJ-347 is the one to go for if your worried about the strength of a dipole centre for 3/8th whips.

I'd prolly go for something more chunky if it were for a vehicle mounting, since there's going to be more movement, also the ends would need tying down to avoid spearing pedestrians, or losing the whole lot to a branch if going off-road..

Whilst not shown the picture, the centre I've bought has a curvature to attaching to a pole upright, using either pre-drilled holes to bolt to the upright, or allow cable ties (not using those), or the jubilee clips I'm planning to use.

The centre was relatively cheap, but look sturdy enough for a base station, and only cost about £5 on eBay, so I'm willing to give it a go here as I'm unlikely to lose the Whips in my garden if it breaks.

Ok Pictures, some a little blurred, but it was bloody cold and I was in a T-Shirt...




Well, you can see my plan now for Packet on 40m...thoughts?

How on earth would you consider using a dipole mobile

TrunkY wrote:

How on earth would you consider using a dipole mobile

I've done it in a 4x4, you fit a roof rack, secure the dipole to it, and tie down the ends to the front and back bumper.  I've also used sloping whips, again, best with the tip tied down.

For NVIS HF Mobile Operation in mountainous areas/heavily forested areas, without VHF/UHF Repeater Coverage.

All base stations in such areas, such as one of the Long Distance Walking Association 100 mile Hikes over 48 hours, you utilise low power NVIS based on the FoF2 Critical Frequency, and get coverage up to 150 miles using skywave, basically if you can see the sky, you can get a signal out.

I am a Member of the RAYNET HF Team, although I've not been active for a long time, I'm gearing up to do it again.

I've run a National Net from a location with Mountains rising very steeply in all directions, with NVIS HF Antennas, Portable, and Mobile, using no more than 10 Watts, often less.

Take a look here.

RAYNET HF Team

Just bit the bullet and bought a set for 40m, I'll swap out the 50MHz halo with it. I'll leave the compromise multi-band antenna as is for now, as I'm not running QRP having a resonant antenna is not so important

If enough of us get some half decent NVIS antennas for 40 we could get some CT action going.

Do let know if a CT something goes ahead on 40 sometime and I'll try to join in if home. 40 is my least efficient permanent element but have made a few contacts between 50 and 300 miles when the foF2 has been high enough.

Related note, on mobile antennas that might be suitable for NVIS I came across an 11m antenna designed for RVs made by Jo Gunn (you should find easily with a quick net search). Something like that probably could be fitted to roof racks of large cars and with a couple of coils to bring it to resonance on 40 and being so low to the ground, it might just work.

My whips arrive later today and the dipole centres are already here. They are a lot better than I was expecting for £7. Any actual outside antenna work will have to wait for a little while however,

Can't believe I fired a couple of you guys up to give it a try.

The existing 40m Dipole Mobile Whips Antenna will be going on my tower soon, need to dig out my Antenna Analyser and tune the SWR before that, but the mounting components are all now here, just waiting on the weather and my son's help to lower the tower to do most of the antenna installation.

However the Roofrack Arrived for the 22 year old Audi S3 Quattro, so once that has been fitted (weather?), I'll invest in appropriate mounts/fixings/tie downs, for a set of 4 horizontal dipoles, at 160m, 80m, 60m, and 40m.

Still no permission from 'er indoors to mount anything in/on the Toyota...

Reading about optimal height above ground (for 40m) I think that I've settled on 15 feet. It could go higher, but I shall try that for now.

Need a dry not so windy day to actually get it out there....

TrunkY wrote:

Reading about optimal height above ground (for 40m) I think that I've settled on 15 feet. It could go higher, but I shall try that for now.

Need a dry not so windy day to actually get it out there....

Yes, my 40m Dipole Mobile Whip will have to be around 16ft or 17ft AGL because of antenna clearance with a 4m Vertical Dipole mounted above.

Depends on a lot of factors but the ideal AGL for a horizontal dipole for NVIS is tad over 1/8th wavelength, above 1/4 or below 1/20th of a wavelength and you lose trajectory or signal strength, then there's the local noise floor, lower usually improves matters, further away from potential sources is also a factor, along with other means of suppressing RFI.

So the 1/8th sweet spots should be as follows: (Note I've used the mid-band frequency as a guide)

40m Band - around 17ft 4 inches AGL
60m Band - around 23ft 1 inch AGL
80m Band - around 33ft 8 inches AGL
160m Band - around 64ft 10 inches AGL

Where signals start to weaken in strength try not to erect below 1/20th as follows:

40m Band - around 6ft 11 inches AGL
60m Band - around 9ft 3 inches AGL
80m Band - around 13ft 2 inches AGL
160m Band - around 25ft 11 inches AGL

(Obviously the new EMF Field Exposure limits should now be adhered to, and Electrical Safety of anyone that may come into contact with an Antenna when transmitting.)

So as you can see, top band will always be a poor performer below 26ft AGL, but also that 40m starts loosing NVIS Propagation above 34ft 7 inches.

Arguably, the best compromise random horizontal dipole for NVIS would be at 30ft 2 inches AGL.

However, we can't all have three ~30ft Supports in the garden for this compromise.

We'd all like a North-South set of 12 supports arranged across a field, able to field four horizontal dipoles at the NVIS sweet spots at 1/8th wavelengths on the NVIS Bands, but this will likely be somebody having a lottery win.

I've bought my ticket.

I got it up today!

I also raised the mast another 15 feet, apparently it may be windy tomorrow.... but you only live once.

Not impressed so far, but the fof2 barely reached 7MHZ today, so it was seriously outperformed by the existing coax fed compromise antenna.

TrunkY wrote:

I got it up today!

I also raised the mast another 15 feet, apparently it may be windy tomorrow.... but you only live once.

Not impressed so far, but the fof2 barely reached 7MHZ today, so it was seriously outperformed by the existing coax fed compromise antenna.

Well, I'd drop it down a bit nearer the sweet spot, wish I could get nearer to it here, but we'll have to see what's possible, as the random dipole is more important, and I don't want these mobile whips coming into contact with any other antenna, as the whips will move about in the wind, so I've got to compromise.

A bit envious that you can actually use yours at this point, but looking at Chilton Ionosounde Maps for yesterday 17th February, the FoF2 only briefly raised above 7 MHz for random ten to twenty minute periods about four times between Mid-day and 6pm, and by the time of your post yesterday was at 5 MHz.

If you were comparing to a DX antenna then the skip distance for 40m was at around 1000 Km, yup the realtime Inosounde Maps give you that data for distances between 100 Km and 3000 Km, so its useful for DX as well.

Ok, tuned my 40m Horizontal Dipole Mobile Whips to 7.050 MHz, which is the GB7CIP Packet BBS Frequency, managed 1.6:1 at about 4ft AGL which should improve below that at 12ft AGL, with each Watson HF-40 Whip ends adjusted at 114.5cm each.

I noted a 5cm adjustment on each whip moved the VSWR ~100Hz, with the maximum extended (flush with the end of the adjuster) give VSWR at 6.900 MHz, so there looks plenty of adjustment to manage 7.200 MHz if required.

However, the SWR Curve is rather sharp, giving about a ~20Hz sweet spot for lowest SWR.

So next time my son turns up, he'll be despatched up the tower with the two whips ASAP, ha.

Ok, my son came round early and put up the 40m NVIS Horizontal Dipole Whips tuned to 7.050 MHz...

It don't look bad...




It's about 15cm shorter than it was, so approx 15ft 6 inches tip to tip, and seems to be a new type of NVIS antenna, an Inverted-U.

I do like to being first, ha.