HF antenna mounting

**DeanoH** · 18th February 2014, 11:38 AM

Originally Posted by Davo

But I think that without a bit of knowledge, i.e. me, you can blow something in your radio. True?

There are two basic requirements for an efficient antenna.

1/. it needs to be of optimal length ie. 1/4 wave length (approximately).

2/. its impedance needs to be matched to that of the transmitter.

1/. is pretty easy to achieve, it's a given length for a given frequency but 2/. isn't quite so easy to attain, for this you need a balun (sort of an aerial transformer) or a manual antenna tuner. Also note that an antennas impedance changes as its length varies from the ideal 1/4 wave.

With a mismatched transmitter/antenna some of the power fed up the antenna feeder is 'reflected' back down the antenna and back into the transmitter. It is VERY important that the transmitter has a good earth so that this power can be grounded out so it does not harm either the transmitter or operator. If you hit the transmit key and have a 'hot flush' you have a major problem.

If the mismatch is bad enough the output transistors in the transmitter may be damaged, though modern transmitters have protection circuitry to help prevent this. Still though 90 watts of RF coming backwards down the antenna can do a lot of damage to person or equipment. Modern Codan and Barrett sets are 100 watt sets.

Even so, with the best impedance matching in the world a crappy antenna isn't going to radiate worth a dam. You could theoretically match a transmitter perfectly into a bucket of water but its not going to radiate too well.

Codan used to make its 'Lunchbox' portable set (6924) with a built in antenna tuner so that it could be efficiently used into a 'random' length of wire, I hired one of these when I went up Cape York in 1979 and it worked very well.

But that was 35 years ago and these sets, like me are getting pretty long in the tooth.

Though a more modern version (6924 Mk II) was built into the 90's ? and are a great little 50 watt basic set. This is the only Codan HF (non marine) or Barrett set with a built in antenna tuner that I know of.

For the average punter though a 1/4 wave of insulated wire with a banana plug at one end to fit into the centre of the antenna socket of the transceiver should get you out of trouble if your 'real' antenna should be lost/damaged.

Deano

**Kevin B** · 18th February 2014, 10:17 PM

The optimal antenna is actually a half wave not quarter, without a balun impedance can be adjusted be raising and lowering the height from the ground, if you mount a half wave antenna have wavelength from the ground impedance will be 50ohms, now reflected power or common mode current as its called can be easily stopped by winding 6-8 turns of coax about 200mm in diameter at the antenna feedpoint and taping it up, this effectively chokes the current from returning down the feed line to the radio, a tried a trusted method Hams have been using for years, alternately if you move the coil choke down the coax to the point were it touches the gound anything above that becomes part of the antenna, the dipole give a horizontal radiation pattern and the coax a vertical pattern, a variation of a highly effective antenna called a Carolina Windum, I have built many of them.

Anyway sorry for the hijack

1993 Discovery 1 3.5l
2" Lift, Cranked HD Trailing Arms
31" Maxxis Mudders
Tons of Radios, Tons of Spotties

**Bushy049** · 18th February 2014, 10:30 PM

Wow this is getting a too technical now, poor bloke just wanted advise on mounting his auto tunner. But I have to say, I thought it was a quarter wave length that was ideal. Hence why a lot of antennas are a quarter wave length like my uhf antenna in my roof and the low band vhf antenna on the bull bar of my work ute.

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**Kevin B** · 18th February 2014, 10:37 PM

Now its gets more technical UHF and VHF are somewhat different to HF in the way the signals propagate, there ony designed for short distance so a quarter wave if sufficient, saying that though the UHF wave length is only about 70cm long so a full size antenna is easy to mount on a vehicle.

Ok sorry to get to deep into it..

1993 Discovery 1 3.5l
2" Lift, Cranked HD Trailing Arms
31" Maxxis Mudders
Tons of Radios, Tons of Spotties

**Bushy049** · 18th February 2014, 11:23 PM

That subject of vhf and uhf is fine, hf has sky waves hence the range. But more the half wave length antenna thing. Is this only in regulation to the hf long wire or vertical antenna?

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**Kevin B** · 19th February 2014, 06:50 AM

It relates yes you can use a half wave vertical antenna, a windmill tower is very close to that on 7150khz, now as for the mobile mount, its impossible to mount a full 1/2, 1/4 wave vertical on you vehicle, even a 1/8 wave at those frequencies is about 2.5 - 3m long its doable but not very practical for some people, that is what the tuner section of your vertical antenna on your vehicle does, it is filledwith a number of coils and relays and as it tunes ( thats the clicking sound you hear) it matches inductance and capacitance and a few other variables to give the best SWR with maximum forward available power and minimal reflected power, in laymans terms it makes your radio "think" there is a longer antenna conected to it, they work but not as effectively as they could, they are convenient but with that comes a losses in the way of power, your radio might be doing 100w but by the time it gets to the antenna it will be much less,

**DeanoH** · 19th February 2014, 10:02 AM

What Kevin says is of course correct, my explanation is to explain in simple terms for a mobile antenna in particular used on VKS-737 frequencies.

Originally Posted by DeanoH

There are two basic requirements for an efficient antenna .................................................. .

should have the rider " for a mobile/portable antenna"

Thanks Kevin, my excuse is that I was trying to keep it simple and in context.

As Kevin states a dipole, 2 1/4 wave elements (a dipole antenna) is the theoretical ideal, but when a 1/4 wave at 8022 KHz (VKS-737 primary channel) is about 9 metres and even longer for the secondary channel this is not a practical antenna for portable or mobile use.

Deano

**Kevin B** · 19th February 2014, 11:38 AM

yeah sorry folks, got a lil offtrack from the original posters question, antennas both mobile and base/portable are such an intricate past of any comms system and often very overlooked, they have been a passion of mine for some years now.

my mobile HF antenna very simply consists of a 8' Stainless steel whip off the back of the vehicle mounted on the spare wheel carrier on top of a tube to bring the base to roof height, from that runs a length of coax to a remote antenna tuner mounted in the back of the truck then to the radio base which is mounted about 30cm away (Remote head is on the dash) there is only about a meter and a half of coax all up (less power loss) the antenna can be bent over and tied to the bullbar if required as well, works very very well, and the main unit being away as far from the enigine bay as possible greatly reduced engine noise while driving.

i have in the past tried a few auto tune antennas from Codan and Barrett doing side by side comparisons with Tapped whips, then best of them all i found was the Tapped whip very simple to use and non Mechanical, there is a company called outbacker that sells them for both HAM radio and RFDS. www.outbacker.biz the beauty is you only have to run coax to them with an autotune you have to run the control cable as well and ideally not side by side.

saying that if you decide to go with the one you have the best place to mount it would be off the back of the vehicle just make sure the actual antenna part is at least level or above the roof line.

Oh and EARTHING, cannot stress this enough, earth Everything, the antenna base the Radio even put extra earth straps from the screws on the radio mounts to ground.

**TonyC** · 19th February 2014, 04:47 PM

Originally Posted by Kevin B

my mobile HF antenna very simply consists of a 8' Stainless steel whip off the back of the vehicle mounted on the spare wheel carrier on top of a tube to bring the base to roof height, from that runs a length of coax to a remote antenna tuner mounted in the back of the truck then to the radio base which is mounted about 30cm away (Remote head is on the dash) there is only about a meter and a half of coax all up (less power loss) the antenna can be bent over and tied to the bullbar if required as well, works very very well, and the main unit being away as far from the enigine bay as possible greatly reduced engine noise while driving.

Hi Kevin,
What is the tuner that you use? I assume it's an auto tuner, given you have a remote head radio.
My Out Backer tapped whip has been repaired twice and I think it's now off frequency. I have often thought of a set up like you describe, but for my use a manual tuner would work OK.

Tony

PS
Sorry for the hijack

**Kevin B** · 20th February 2014, 09:41 AM

Originally Posted by TonyC

Hi Kevin,
What is the tuner that you use? I assume it's an auto tuner, given you have a remote head radio.
My Out Backer tapped whip has been repaired twice and I think it's now off frequency. I have often thought of a set up like you describe, but for my use a manual tuner would work OK.

Tony

PS
Sorry for the hijack

Tony,

yes it is an auto tuner, the model is an LDG Z100, the model before the one in the following link, but with the same Specs Products LDG Electronics ,

Kevin

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