Shortest antennas for 39 MHz and 151 MHz?

[Stringer619]

I need to buy two antennas for the top of my sedan. One for CHP (39MHz) and one for CALFIRE (151MHz). If I understand the cutting charts correctly, a 39MHz antenna is something like 4-5 feet. I can't do that.

So instead, what are some alternative options? I can't really go longer than 2 feet. I'm willing to take a "less than best" antenna because anything has to be better than a little handheld whip antenna.

 

[nick1427d]

On 39 mhz for any solid reception your shortest might be around 50''. Now if CHPs signal is fairly strong where you're at you might be able to get away with something shorter.

If you're using this one antenna for both 39 and 151 how about buying a unity gain whip and cutting as long as possible (for 39mhz). You can get these super cheap on ebay and other sources. Most use NMO style mounting so you can use a permanent or mag mount. If it doesn't work you're only out the money for the antenna.

For example where I'm located I can use a short unity gain for vhf and still get good reception on 800 systems and some UHF. This is because most signals where I'm at are close/strong. If what you're attempting to monitor is weak and or you have a lot of obstructions and hills well then you're kinda screwed.

 

[Stringer619]

I had a unity gain whip cut to 151MHz and I was able to pick up CHP good enough to make out what was being said, but there was a lot of room for improvement. Since then I cut it down to an 800MHz antenna for a different scanner. Ironically CALFIRE (151MHz) came in like garbage unless I was within a mile or two of the scene in the mountains (wildfires).

If a 1/4" wave is 50" (example number), then if I cut it down to 25" would that make it 1/8 wave and would that work better than say, a 28" whip? I guess what I'm asking, is do I need strictly base my measurements on halves?

I think I could go for a shorter unity gain antenna for CHP 39MHz, but is there a "gain" antenna for 151MHz? Cause I need to boost the hell out of 151MHz.

Also, if you have the option of a short (roughly 2ft) antenna on the metal roof of a sedan OR a 3ft antenna on the sedan trunk lid (which drops 3db per a diagram I saw once, compared to the roof), which would you choose? Would the additional 1ft negate the consequence of losing 3db? Both would be permanent NMO mount.

 

[n5ims]

How about getting a Larsen NMO34C (unity gain) for your VHF-Low needs and a Larsen NMO150C (3 dBd gain) for your VHF-Hi needs. Mounting them on either side of your trunk using those "L" bracket NMO mounts. The two antennas will look similar, with a fairly small coil at the bottom and a thin whip of close to the same length. The trunk mounting should give you better clearance than a roof mount and should work pretty well.

They're shown in this .pdf along with that chart you probably saw with the placement loss numbers. http://www.pulseelectronics.com/download/3598/larsen_amateur_catalog_vol_1

 

[Stringer619]

Excellent suggestions! I especially like Larsen based on past experiences. I will certainly consider those antennas, but at 49 inches that is VERY long. Any suggestions for a shorter alternative? Albeit likely with reduced performance, but just out of curiosity.

Larsen - 34-40 MHz Base Loaded 1/4 Wave Antenna, Chrome
Larsen - 144-174 NMO 5/8 Wave Antenna, Chrome

I have two roughly 4" 800MHz whips on my roof and haven't had any problems, but I have to wonder how likely vandalism/theft would be with larger and easier to access antennas. I can just imagine parking at a Wal-Mart and someone tweaking them, but I guess that comes with the territory.

And yes that was the chart I saw.

 

[Stringer619]

If I could find something like this with 3db of gain or higher, I'd be very happy, regardless of price. I've always thought the Phantom antennas looked sweet.

Laird 150-168, Phantom Mobile Antenna

 

[nick1427d]

I had a unity gain whip cut to 151MHz and I was able to pick up CHP good enough to make out what was being said, but there was a lot of room for improvement. Since then I cut it down to an 800MHz antenna for a different scanner. Ironically CALFIRE (151MHz) came in like garbage unless I was within a mile or two of the scene in the mountains (wildfires).

This is probably due the mountainous terrain.

If a 1/4" wave is 50" (example number), then if I cut it down to 25" would that make it 1/8 wave and would that work better than say, a 28" whip? I guess what I'm asking, is do I need strictly base my measurements on halves?

From my experience and knowledge which isn't much, we don't measure wavelengths with 1/8 wave. Wavelengths are measured with:

Full
3/4
5/8
1/2
1/4

Using a Full wavelength antenna would be entirely too long, therefore we use fractions of the wavelength.

I think I could go for a shorter unity gain antenna for CHP 39MHz, but is there a "gain" antenna for 151MHz? Cause I need to boost the hell out of 151MHz.

I would try a 5/8th wave for 151 which would be right around 4 ft. The propagation characteristics of a 5/8th wave is dedicated more upwards at an angle, kind of like a beam more precisely around 15 degrees. However don't be surprised that at this angle with the mountains it get worse. "Gain" doesn't simply mean you "gain" a signal.

Also, if you have the option of a short (roughly 2ft) antenna on the metal roof of a sedan OR a 3ft antenna on the sedan trunk lid (which drops 3db per a diagram I saw once, compared to the roof), which would you choose? Would the additional 1ft negate the consequence of losing 3db? Both would be permanent NMO mount.

Centered on a metal roof will give you the best performance hands down every time. Again it's not the "gain" or "loss" it's the fact that the metal roof creates a ground plane which is needed to create proper propagation.

Now I'm feeling that you're quite confused as to a route to go, and I'm thinking height will be your deciding factor am I correct?

You can't really compare a 2ft to a 3ft no matter where the location on the car because they will be cut different for the frequency. You cut the whip for your frequency. It really only needs to be exact if you're transmitting. If receiving then just get it close.

If it's for 151 then it's either a 1/4 wave or 5/8. I'd try a 1/4 nmo on the center roof, be about 19.5" you won't get "gain" but you won't have to worry about height. It's cheap and again if it doesn't work you could always sell it or keep as spare. A 5/8 will be 48.8", can be found used on ebay at times, make sure it wasn't already cut shorter for like the 160 or 170 ranges.

Online Conversion - Frequency Wavelength Calculator
Marc's Technical Pages: Choosing the correct antenna
Marc's Technical Pages: Antenna Gain Explained

 

[nick1427d]

If I could find something like this with 3db of gain or higher, I'd be very happy, regardless of price. I've always thought the Phantom antennas looked sweet.

Laird 150-168, Phantom Mobile Antenna

Although they do look very slick and "covert" these antennas give similar results to a 1/4 wave unity gain at half the cost (at least).

Laird Tech QW144 - $9.95 : The Antenna Farm :: , Your Two Way Radio Source!

 

[W2NJS]

Save yourself a lot of time and go to a manufacturer's online catalog like the one available from Comtelco.
You'll find several varieties of the antennas you've been discussing here. You can buy from Comtelco via an 800 phone call and the prices will be discounted from the catalog's prices.

 

[reconrider8]

has anyone thought about a spectra? its not so long and it will run lowband through 800 pretty good across the board

 

[Anderegg]

I have an NMO mount 39 MHz cut antenna for you if you want it.........just keep it in your trunk and pop it on for pursuits and such.

My current set-up works great for CHP...can you spot my low-band whip? ;-)

Paul