Mobile HF antenna’s

Can anyone suggest a reasonably priced mobile antenna for HF work?

As a complete novice to HF (and my interest being in mobile RAYNET work) any suggestions for a "starter" antenna would be appreciated.

I have also heard interesting reports recently about the use of top band mobile over distances up to 50 miles or so, however the antenna design theory horrifies me for that wavelength! Any suggestions?

Hi Jim,

I've had some experience experimenting with Mobile NVIS, and must echo Greg's comment regarding what you might consider to be 'mobile'.

Quickly deployed Portable stations from vehicles, especially with vehicle mounted radios, are sometimes referred to as being Mobile NVIS by those that use them. Truly Mobile NVIS stations, as I've mentioned on another thread, will always be a difficult compromise between 160m and 40m, since the antennas will always be less efficient due to their composite size. Available bandwidth, particularly on 160m, will be the limiting factor for moving away from QRM/N.

Reference to the ICEPAC predictions available from the RAYNET HF website, along with the links to real-time ionosounde information will quickly establish the current Critical Frequency, since the useable band changes over the 24 hours. Greg has even been known to produce ICEPAC predictions on request, Hi!

40m at it's current band limits could be difficult in terms of QRM, particularly when establishing a Net or trying to QSY, so efficient antennas must be the priority, along with good Net Control. The ability to 'Command' a frequency with sufficient power is desirable.

5MHz cannot legally be used Mobile.

80m on average has a higher noise floor than 40m, so you may need to use more power, and a Mobile Antenna may have only 40-50kHz of bandwidth within a 2:1 VSWR curve.

160m on average has a significantly higher noise floor than either 40m or 80m, and there are limitations on the maximum PEP available under BR68, a Mobile Antenna might have only 10kHz of bandwidth within a 2:1 VSWR curve.

Mobile to Mobile Emercomms using NVIS HF may even be unreliable (this is an area that the RAYNET HF Team really needs to investigate), depending on the power used, topography, the antenna installations, and variations in the ground. So the first consideration should be establishing a Portable Control Station with the most efficient antenna available for the circumstances, to be able to hear all the Mobiles. I've mentioned use of sufficient power to maintain the contact, and with the limitations on what can be effectively achieved with a Mobile antenna, this might mean initial use of 100W at the Control as a starting point for establishing Emercomms.

Certainly the power requirements for a Portable Station should be addressed early on, since the current drain on batteries or a 240vAC generator with low amperage 12vDC could be high. It is possible that a high amperage 12vDC generator could be a better solution.

Returning to the problem of a truly Mobile installation, the topography is something we're trying to overcome, and so is a constant for the Emercomms Duty.

Mounting a HF tilted whip over the vehicle bodywork isn't the easiest option on most vehicles, since any modifications might affect the re-sale value if nothing else, so what about having the antenna at the back of the vehicle over the road?

Some good results have been achieved by the RAYNET HF Team using tilted centre loaded whips, mounted as high as possible on the vehicle above the road, with the consideration of mechanical longevity and other road traffic being a priority for the installation. One point to note is that the road or 'ground' over which the tilted whip is being used is constantly changing, particularly with height ASL, so it is worthwhile tuning the whip on-site rather than relying on what you've achieved at home.

Some commercial solutions such as those provided from Australian companies, use either the vehicle bodywork (1m above roof) as the reflector for long tilted whips, or mount a loop on a roofrack. These offerings work well (we're told) for NVIS, but are expensive, with proprietary transceivers and auto ATU's being a part of the package.

The idea of using the vehicle body as the ground reflector has to be looked at in more depth, and tests are planned using a composite dipole mounted on a roofrack, with the antenna at least 1m above the vehicle bodywork. If this is more effective, it would be a convenient installation, as long as the increased vehicle clearance is observed at locations with overhanging trees or vegetation.

The magnetic loop with motorised tuning capactor may also be a convenient solution, although still a composite antenna (requiring a very big air spaced capacitor). I'm told that some GPO vans used to have loops near 5MHz mounted within the roof many years ago, but how effective these were is not well known.

160m Mobile is probably best when using a vertical whip (another area for investigation), but again consideration must be given to the mounting in terms of mechanical longevity, a solution using a magnetic tri-mount and 3/8 threaded whip is the preferred option considered here, but I'm told that some of these tri-mounts are difficult to remove from the roof!

The antenna used here with some success over difficult terrain is the Watson WHF range of whips, tuned in the tilted position on-site, but really any centre loaded whip would have similar results. I've used a Diamond Heavy Duty mount, which is suitable for large VHF or HF whips, and intend to invest in a magnetic tri-mount for 160m.

You'd need centre loaded whips for 40m, 80m, and 160m to follow the change in critical frequency over 24 hours.

Hope this helps.

Hi,

Preliminary tests here using 40m centre loaded whips configured as composite horizontal dipole at 1m AGL, with a ground reflector consisting of aluminium baking foil (ahem), show that tx/rx exhibit a 20-25dB loss in signal when compared to a reference 40m half-wave dipole @ 16ft AGL, without a ground reflector.

This is quite encouraging, since use of 40W into these antennas at my QTH in Shrewsbury, gave signal reports at the receiving station at Whitley Bay (M5ADZ) of 59+10 for the reference dipole, and 57/8 for the composite dipole, in quite noisy conditions. The FoF2 being around 7.9MHz at RAF Fairford.

Reasonable signals were received from stations while using 100W into the 80m composite dipole @ 1m AGL, during the recent RAYNET HF Team Net, although not everyone received good copy, I was a good signal 20 miles down the road in Welshpool (GW4BVE), and also received well on the Isle of Man (GD3LSF). The foF2 was 'perhaps' a little high for 80m, at ~5.6MHz, and indeed the Net moved up to 5MHz for comparison tests before it closed down Inter-G.

Although I intend to continue tests @ 1m AGL, without a ground reflector, I am encouraged enough to consider the options for future mounting of this antenna on my 4x4 for further testing.

I've not heard of anyone undertaking NVIS tests using a slinky, but this is certainly lateral thinking, and might well be worth someone experimenting to find out whether it works. The configuration used on the BBC2 programme must have been best for short skip DX to manage 14MHz to the Canary Islands (didn't see the programme), but there's nothing stopping anyone exploring use/configuration for NVIS Inter-G. I'd be interested in anyone's results.

I now have the tri-mag mount and a 160m centre loaded whip, so am threatening to operate Mobile in the early hours at some point, I hope there will be others awake within a 50 mile radius to undertake tests, Hi!

Thanks.
Next time I'm in the members area of the ARRL site I will take a look.