Best Moblie Antenna for OTR Truck

[fourstringburn]

I drive a macks and freightliners daycab. The 5ft firestik or skipshooter toploads work very well behind the cab , via vicegrip pliers mount because of slipseat operation. I have a stargunn big coil also. I cant tell any differnce in the 3. The main thing is tuning it for resonance NOT lowest swr.

Have you been able to do an A/B test between mirror mounting and behind the cab?

I've done it with the trucks I drive and noticed quite a difference even on receive. Day cabs can have more space between the rear cab and trailer over some sleeper trucks so you might get away with it better than most.

Bottom line is get your antenna in the clear and as high as possible and away from metal except any metal underneath for best performance. This holds true for base antennas too.


As for shortened mobile antennas, what you said is usually correct. You're not likely to notice much difference if they are all tuned up properly between different manufacturers.

 

[fourstringburn]

Most radiation occurs in the upper half of this antenna, but unfortunately most people wouldn't know a decent antenna if it slapped them in the face. Most folks are using bottom load antennas.

I like that antenna, I'm going to copy that design and use it for my little mobile CB station.

 

[Beetle]

Rabbi - when that picture popped up, I thought the antenna was actually attached to the front of the vehicle in that position. Then I scrolled down and everything was fine! But there, for a second, .....!

 

[HARDROCK]

Have you been able to do an A/B test between mirror mounting and behind the cab?

I've done it with the trucks I drive and noticed quite a difference even on receive. Day cabs can have more space between the rear cab and trailer over some sleeper trucks so you might get away with it better than most.

Bottom line is get your antenna in the clear and as high as possible and away from metal except any metal underneath for best performance. This holds true for base antennas too.in


As for shortened mobile antennas, what you said is usually correct. You're not likely to notice much difference if they are all tuned up properly between different manufacturers.

I have tried both . The back of the cabs are metal so it grounds better than the mirror arms .

The most important issue is Tuning for resonance and not lowest SWR. By using a matching transformer at the antennas feedpoint. The tx and rx is much better. I have been in the cb hobby for 30 yrs and always have tuned for lowest swr. Until recently i tried the matcher and WOW it makes a huge difference. I run a 1990 philly cobra 29 ltd, and like most i always had the opinion that the 29 did not recieve like the exports. Until i was given the above advice by g3sto on tuning mobile antennas. The 29 recieves better than the exports without the noise. I posted here because i wanted to share what i have learned after all these years.

Tony 73

 

[flatlander89]

Hard rock,what are you using for a matching transformer if you don't mind me askin

 

[HARDROCK]

It is a 2.5.1 ratio of what the antenna is. Made by k4poz he has the pics on his site.http://k4poz.com/match/index.html

 

[HARDROCK]

It is a 2.5.1 ratio of what the antenna is. Made by k4poz he has the pics on his site.http://k4poz.com/match/index.html

 

[HARDROCK]

View attachment 19192

View attachment 19192

 

[fourstringburn]

View attachment 19192

Why would you need a matching device for a manufactured CB antenna?

Those transformers are for HF antennas that have feedpoints below 50 ohms. I use them on my HF screwdriver mobile antenna because as I get down below 10 MHz, the feedpoint impedance drops significantly as low as 12 ohms.

A commercially made CB antenna like Wilsons, Firesticks, etc. are already matched within the coil creating inductance that should be at or very close to 50 ohms so no matching transformers are necessary.

If your mobile CB antenna is less than 40 ohms at the feedpoint with an analyzer, I would start looking elsewhere for your antenna problems..

 

[fourstringburn]

Here is a Pic of my HF antenna which features true top loading in the form of a Capacitance Hat. Also you can see a properly configured common mode choke made with 7 turns of coax thru a Mix 31 3/4 inch split bead ferrite.

Although hard to see, above the choke is a coax fed UNUN matching transformer box patched in right at the antenna feed point and that is selectable for 12 ohm feed point impedance's typically found on 40 meters and below and selectable for 25 ohm impedance's for 20 meters up to 10 meters. In most cases, 10 meters and up the transformer is virtually invisible to the effect of any matching in this manner. This includes the 11 meter band as well.

 

[flatlander89]

Thanks for the info,Hardrock

 

[HARDROCK]

Thanks for the info,Hardrock

You are welcome flatlander.

 

[HARDROCK]

Why would you need a matching device for a manufactured CB antenna?

Those transformers are for HF antennas that have feedpoints below 50 ohms. I use them on my HF screwdriver mobile antenna because as I get down below 10 MHz, the feedpoint impedance drops significantly as low as 12 ohms.

A commercially made CB antenna like Wilsons, Firesticks, etc. are already matched within the coil creating inductance that should be at or very close to 50 ohms so no matching transformers are necessary.

If your mobile CB antenna is less than 40 ohms at the feedpoint with an analyzer, I would start looking elsewhere for your antenna problems..

Good looking setup on your truck.

You missed the point just like i did for a long time.
An effiecient mobile antenna will be between 18 and 36 ohms impedance at feedpoint.
An ineffiecient mobile antenna may be 50 ohms at feedpoint.

.
Most amateurs have become used to tuning for a minimum indicated SWR. This is meaningless in a mobile installation. WHY? An efficient mobile whip is unlikely to be 50 ohms. An inefficient one may be! Minimum SWR is NOT an indication of maximum radiation; it simply means the tuning device has found a point where it thinks the antenna exhibits 50 ohms but, to achieve this the voltage and current will not have the correct phase relationship for optimum radiation.

This is from g3sto homebrew mobile antenna site.
He is Dead on!!!

 

[fourstringburn]

Good looking setup on your truck.

You missed the point just like i did for a long time.
An effiecient mobile antenna will be between 18 and 36 ohms impedance at feedpoint.
An ineffiecient mobile antenna may be 50 ohms at feedpoint.

.
Most amateurs have become used to tuning for a minimum indicated SWR. This is meaningless in a mobile installation. WHY? An efficient mobile whip is unlikely to be 50 ohms. An inefficient one may be! Minimum SWR is NOT an indication of maximum radiation; it simply means the tuning device has found a point where it thinks the antenna exhibits 50 ohms but, to achieve this the voltage and current will not have the correct phase relationship for optimum radiation.

This is from g3sto homebrew mobile antenna site.
He is Dead on!!!

Your missing the point and have a lot to learn as opposed to memorizing what you read.

Your mobile CB antenna is already inductive matched and needs no other matching. Those matching devices you linked are for a mobile HF screwdriver just as the website itself suggests.

When we are talking amateur bands well below the CB band, the proper antenna size for the wavelength of a frequency is far too short and even gets shorter as frequencies decrease. To compensate, we use inductive coiling to bring things back up to a usable range resulting in the feed point impedance dropping and increasing the reactance and swr. Then we fix that by adding some feed point inductive matching like using shunt coils or transformers like the ones K4POZ sells on his website to get the feed point impedance back up or close to 50 ohms and should get our reactance value lowered down to near 0 which will result in a low SWR. This will be as resonant as we can get. Some mobile HF antennas have enough coiling to match up good but these lose more efficiency because the more coil used in the antenna, the more radiated losses.

True low SWR isn't always an indicator of maximum radiation but it is usually close. An antenna analyzer is one of the best way to find out. In simple terms, on an Analyzer, you want the Reactance represented by X to be at or near zero. You will find that if you able to get there, the feed point impedance represented by R should be very near 50 ohms and your SWR should be very ideal.

There are many variables to mobile installs to say feed-point impedance's are resonant perfectly at 18 to 36 ohms. With low impedance's fed with 50 ohm coax, your suggestion represents a current loss right at the feed point.

Do you have or have access to an antenna analyzer? measure your antenna and take a pic and posted it here so we can see what it's reading.

I see no links to G3STO, I would suggest reading K0BG website that is renowned for the best source of Mobile HF operation information.

http://www.k0bg.com/

 

[rabbiporkchop]

Rabbi - when that picture popped up, I thought the antenna was actually attached to the front of the vehicle in that position. Then I scrolled down and everything was fine! But there, for a second, .....!

That was just my test jig mounted on my tire carrier for basic ballpark tuning the coil. Final tune was to be done on the mirror arm of a Freightliner Classic.