Antenna Mounting Solutions - and problem solving

[L81ker]

Hi Everyone,
I've been searching without too much luck to find antenna mounting solutions for amateur radio. I need to get 2 antennas on my '05 freelander TD4 without compromising the resale value.

I'd like to start this new thread to specifically cover the options available.

Here's what I have found so far:
FACTORY ROOF BARS - These are often not grounded, and are also plastic coated. They are often fitted using screws tapped into plastic...

PSM-1 3/8 MOUNT - perfect solution if you don't mind drilling a rear quarter panel
http://www.4x4cb.com/public/show_faq_info_preview.cfm?FAQID=289

MAG-MOUNT -great temporary solution as long as you don't have a fibreglass / aluminium /soft top roof!

MIRROR MOUNTS -these might be great on a landy, but so far don't seem a good idea for Discos or Freebies - not sure about rangies. For CB'ers there are ground plane versions which basically turn the CB antenna into a vertical dipole by adding a second tuned whip under the mount.

MAKE A PLATFORM -I've seen several solutions where a platform for the antenna is made either on a roof rack or else in place of the rear-mounted spare wheel. The platform is made of sheet metal on plywood, or from tubing or section. This is a great solution from an antenna point of view, but will add wind resistance and noise - and can be a bit ugly.

TECHNICAL STUFF for the LESS TECHY
I'd like to have a comprehensive post from someone eloquent to explain the vagaries of antenna theory regarding mobile applications, without getting into all the theory that might be too confusing from other sources such as ARRL or RSGB.
E.g. what is coax, why a rig needs to see (usually) 50 ohm's impedance, and how an antenna is tuned to resonance to provide that 50 ohms.
E.g. what is radiation resistance, and why "other things" can detune the antenna - or cause a loss of radiation 'capability'
E.g. what is a radiation pattern and why choosing your mounting point has an effect on the lobe...

Kind Regards
Clive

G1WZM
(ex 9V1AZ, VR1WZM, OH2FBK)

[bvudzichena]

After two years of trying to get my ZW license transfered to SA, I've given up and am (re)sitting my Class A exam in May and have been looking at similar solutions for my DII.

Don't forget that you don't want too much distance between your ATU and your aerial(s)...

I honestly don't know how I'm going to do this and am also hoping someone "in the know" will chip in.

[Marc Lurie]

I honestly don't know how I'm going to do this and am also hoping someone "in the know" will chip in.

Do I qualify? :D

Hi Clive,

FYI, I have been in the two-way radio industry since 1991, and I have sold and installed literally hundreds of rados in vehicles.

What antennas are you wanting to install? If you're looking for a quick and easy VHF antenna for the 2m band that won't damage the car, then go for an on-glass type antenna. This antenna has a small pad that sticks onto the inside of the window, and the antenna sticks on the outside. The coupling is done capacitatively. They're not ideal, but if it's for repeater work, then they're perfect. When yu remove it you peel off the sticky pads, and there's no trace of the installation. They're available in UHF too for the 70cm band.

One thing that you have to bear in mind is that a mobile installation will ALWAYS be a compromise, never a perfect solution.

I'm always in favour of drilling a hole for a VHF antenna though.

For HF antennas there are several options too. The most succesful HF installation I ever did was on my own Defender. I mounted the ATU on the rear of the roofrack, and attached that to a helically wound whip that I had specially built. Being entirely above the roof of the vehicle, the antenna performed like a dream.

A screwdriver-type antenna is an option. High Sierra in the US make one that I've used quite succesfully. mounted UN-style on the front bumper.

The Moonraker AT-230 antenna system is excellent, but pricey.

SGC (http://www.sgcworld.com) make a QMS (Quick Mount System) that uses magnets and rubber straps to attach an ATU and antenna to almost any vehicle.

[bvudzichena]

Do I qualify? :D



I was hoping you would join the conversation.

We used to sell High Sierra antennas. They are goooood :D

[Marc Lurie]

TECHNICAL STUFF for the LESS TECHY
I'd like to have a comprehensive post from someone eloquent to explain the vagaries of antenna theory regarding mobile applications, without getting into all the theory that might be too confusing from other sources such as ARRL or RSGB.
E.g. what is coax, why a rig needs to see (usually) 50 ohm's impedance, and how an antenna is tuned to resonance to provide that 50 ohms.
E.g. what is radiation resistance, and why "other things" can detune the antenna - or cause a loss of radiation 'capability'
E.g. what is a radiation pattern and why choosing your mounting point has an effect on the lobe...

That's not as easy as it sounds, but here goes...

1) What is coax? Coax is a specialised type of cable used mostly for radio installations. If you look at it carefully you will see that there is an inner conductive core surrounded by an insulator, and then an outer contuctive core surrounded by the outer sleeve. The ratio of the diameter of the two conductors as well as the nature of the internal insulating material is critical for coax to work properly. All radio waves are Alternating Current (AC). At high frequencies AC acts kinda weirdly when it travels along a conductor, and all sorts of currents are induced in nearby metallic objects (In the case of coax, the currents are induced into the outer shield of the cable. In order to effectively transfer energy along the cable, the cable has to exhibit a characteristic impedance. Think of impedance as a kind of special electrical resistance that applied only to AC. If the impedance of the cable is matched to that of the source (the radio) and the load (antenna) then there is very little "AC resistance" and efficient transfer takes place.

2) Why 50 Ohms?
Radio systems could be designed to operate at any impedance, but for many highly technical reasons they are most efficiently designed at a characteristic 75 Ohms. However, 75 Ohms was difficult to use in the early days of radio, and 50 Ohms became the standard. There's not THAT much difference between 75 and 50 Ohm systems, so there's little reason to change the standard now.

3) How is an antenna "tuned" to 50 Ohms?
Well, there's many ways to skin a cat, and many ways to design an antenna. Many antennas exhibit a natural 50 Ohm impedance, some exhibit a 300 Ohm impedance that is brought down to 50 Ohm by inserting a special type of 6:1 transformer. Others are "tuned" by adding capacitors or coils. There are as many ways of tuning an antenna as there are different types of antennas. I'm afraid there's no non-techy way to explain antennas, except in the very simplest of terms: An antenna is the device that allows radio waves to enter space as efficiently as possible.

4) Radiation resistance and detuning
As I mentioned earlier, AC induces current in nearby objects. The car body is such an object. Car bodies "block" radiation or reflect radiation in unpredictable ways.

5) Radiation patterns
Most vehicle antennas are designed to be omni-directional. (They radiate equally badly in all directions ;)) If you look down on the vehicle (and could see the radio waves leaving the antenna) you would see what would look like a pebble dropped in a pond with radio waves spreading out from a central point in all directions. However, if you looked at the system from a slight angle you would actually see what would look more like a doughnut shape. That is the azimuth radiation pattern. There is no radiation directly above the antenna, and most of the radiation will be in a clear band of angles (say between 20 and 70 degrees from the horizontal). This is the angle of radiation. In most cases, you would prefer a lower angle of radiation, in order to focus as much energy at the horizon. Focussing energy at a point above the horizon is OK if you want to talk to an aircraft or the space shuttle, but wasteful if you're trying to talk to another vehicle that is on (or below) the horizon.

Low-tech enough? :D

[Marc Lurie]

We used to sell High Sierra antennas. They are goooood :D

They sure are. I never really sold them because I require frequency agile antennas that can tune in milliseconds without an operator.

If I were looking at a purely ham-use antenna, I'd seriously look at one of those again.

[Rex Hunt]

Good write up Marc

Well done!

Rex

[L81ker]

Thanks for a well written and very eloquent post, Marc!

My thoughts when creating this thread -we have the technical knowledge in the forum, and we have some non-tech people who need help - how can we techie types share the knowledge in lay terms.
I've seen posts asking if a short circuit antenna coax connection would be a problem, because the user had blown 2 rigs up already...

I'd like to see this thread continue like Marc has already started, and I'll try and add some hand-drawn contributions of my own as well.

---
To Bvudzichena with a disco 2 - I haven't solved my freelander problem yet., here's what I'm thinking:
Solution 1: fit a PSM-1 on driver's side rear quarter panel, about 3" below the roofline. It has a 3/8" stud mount, so use a bayone quick disconnect (they are about 3" long) and fit whatever antenna I want above it. E.g. For 2M a 5/8, or other tuned antenna with a centre loaded coil will work fine and the radiation pattern should be useable.
The PSM 1 has a ground and a "live" bolt - if I fit a 9' whip, I could put a small antenna tuner inside the rear quarter panel connected to that 'live' bolt and to chassis ground. There's even room for a un-un or a balun or a capacitor and a coil to aid tuning to get on top band.

Solutions 2: remove the spare wheel, fit a bracket made of stainless steel to the bolts behind the wheel and ground the bracket well. Refit the spare wheel. The bracket should have a trianglular shape at its fixing point so it uses all the studs for the spare wheel mounting (don't wanna distort the alloy wheels...)
The bracket above the trianglular fixing should be completely flat, about 1/4" or 5/16" thick, and rise vertically straight up to about 4" below the roofline, and then be bent over to provide a horizontal platform. Check that the resistance measured with a dvm or ohm meter is less than 1.2 ohms, between bracket and battery negative.
The tricky bit is that the bracket is going to be close to resonant on e.g. 2M, and might easily form the bottom of a dipole "by accident". We could take advantage of this and deliberately cut the bracket length to be an electrical 1/4 wave on 2m, and fit a 1/4 wave whip above - forming a 2m vertical dipole. This kind of bracket is definitely going to cause VSWR problems on some frequencies!

Again, for HF we could fit a tuner inside the rear door with a short wire to the antenna whip - put a long as possible whip and away we go.
---I don't have the equipment to make test brackets like this, is someone willing to have a go?
---does anyone have antenna modelling software to try this and see what kind of bracket lengths will be 'problematic' for CB and other 'pre tuned' antennas?

I will update this thread with all my findings from my own experiments.
Kind regards, and thanks for all the support
Clive Turner
(MIET)

[Marc Lurie]

Clive, Rex, thank you for the compliment. :)

I've attached two photographs of my mate Gary's (ZS6YI) mobile installation. What do you think? :D

He used to have a Jaguar XJ-S convertible with a 1kW amplifier in the boot, but got rid of the Jag when he bought the Roller.

He's not struggling for money is our Gary... :D

[Marc Lurie]

Here's a photo of my most recent erection ...;)



She's a 19 element rotatable Log-Periodic from 3-30MHz. Total weight around 9000kg. Unfortuantely she's in Luanda, Angola and not in my back yard. :D

[bvudzichena]

...and this is how to do an HF antenna installation in a 110.

The matching rig is an icom 706.

[Rex Hunt]

I did the same job with a 'Smart Tuner to a 30ft whip (yes honestly) for use on all bands.
Whip mounted either on top of 30ft RACAL telescopic mast on rear bumper , or on gutted military side mount arms.
Feed TX/RX Yaesu FT897
QTH.....Exmoor,Dunkery Beacon, North York Moors, Malham, North norfolk Coast, Beltring, and more Scout Jamborees than I can now list!

but

Horses for courses I guess.....bit OTT for CB!

Rex

[timbott]

I did the same job with a 'Smart Tuner to a 30ft whip (yes honestly) for use on all bands.
Whip mounted either on top of 30ft RACAL telescopic mast on rear bumper , or on gutted military side mount arms.
Feed TX/RX Yaesu FT897
QTH.....Exmoor,Dunkery Beacon, North York Moors, Malham, North norfolk Coast,

Tell me more Rex, sounds interesting:)

cheers, Tim