mobile for a base antenna vs real base antennas

[blueangel-eric]

I was considering getting groundplane kit and putting 5/8wave mobile up on a mast for a base antenna but will it work as good as a high gain base antenna? mobiles are rather small in size compared to the more sophisticated high gain base antennas.
I use a Maxrad Maxscan type antenna up there for the base scanner but not sure how good it would measure up to my scantenna. my scantenna is good but at my new location it refuses to get the railroad hot box detectors. my arrow antenna style Jpole does. and they say jpoles aren't all they're cracked up to be. btw i've moved the antenna all around and up and down the mast and now at the top and no go. i'm trying a homemade 2 bay dipole array next. i wonder if my LMR400 would work any better on the scantenna on the RR band.

 

[n4yek]

...i wonder if my LMR400 would work any better on the scantenna on the RR band.

Possible, it depends on what you are using now.

 

[k9rzz]

I can't answer any specific questions, but a great way to answer them yourself is to have more than one antenna up at a time and switch between them. Then you will KNOW what works best in your location. You can never have too many antennas !!

 

[Don_Burke]

If a J-pole is beating the Scantenna, you may have a signal desensing the receiver connected to the Scantenna.

It could be that the J-pole is not picking up the desensing signal very well and is acting as a preselector.

 

[blueangel-eric]

If a J-pole is beating the Scantenna, you may have a signal desensing the receiver connected to the Scantenna.

It could be that the J-pole is not picking up the desensing signal very well and is acting as a preselector.

my radio is the BC785D and as far as i know it doesn't desense. now if it were a radio shack.......
Besides the weather stations and even a long distance 2 meter repeater comes in fine so i believe desense isn't the problem. I'm not sure what to think but even my portable base station with a single dipole doesn't get it to well in that area either. I wonder if the Jpole has just enough gain to get them. maybe i should be a jpole like that for the scanner and have matching sets up there.

 

[KC9JUM]

That arrow J-Pole is tuned with GAIN for VHF. Use the J-Pole and throw away the scanner antennas.

Inside of more Fibre Omni antennas. You will find a variation of the J Pole theme

 

[AK9R]

That arrow J-Pole is tuned with GAIN for VHF.

Gain compared to what?

Gain numbers, when properly stated, include a reference. For antennas, gain is usually stated as dBd for reference to a dipole or dBi for reference to an isotropic radiator. The gain of a dipole antenna compared to an isotropic radiator is approximately 2.15 dBi.

So, be careful when looking at advertised gain numbers. One antenna could be advertised with a gain of 3 dBd and another with a gain of 5 dBi. At first glance, the second antenna would appear to have more gain. In reality, the gain of these two antennas is roughly the same. If the manufacturer won't say what reference they used, take their gain claims with a large grain of salt.

There's nothing magic about a J-pole antenna. The common J-pole is an end-fed half-wave antenna. As such, it has the same gain as a dipole (also a half-wave antenna). In other words, the gain of a common J-pole is 0 dBd.

I like J-pole antennas (and I have one of the Arrow dual-band J-poles). They are very useful because they don't require a ground plane like a 1/4 wave vertical does. When properly designed, they are reasonably broad-banded. And, when properly-tuned, they seem to be great performers. But they still don't have any gain compared to a properly-tuned dipole.

Oh, most commmercial fiberglass antennas are co-linear designs that get their gain by stacking tuned sections that are coupled with coils. The old Phelps-Dodge (now RFS) Stationmaster VHF antenna is one such co-linear design that has been used in hundreds of base station applications. It has a gain of about 5 dBd and it's almost 20 feet long. You don't get something for nothing when it comes to antennas.

 

[wwhitby]

I've got a 900Mhz mobile antenna with a pizza pan for a ground plane in my attic right now. I'm using it as a dedicated antenna for ATCSmon.

Its only temporary, since i'm planning on replacing it with homebuilt phased yagis. However, it does work fine.

Warren

 

[kb2vxa]

First of all a mobile antenna can give only so much gain due to limited size. Can the 2 bay dipole idea right off the bat, too large for too little gain and you have the power divider (phasing harness) to consider, too much trouble for too little results. You seem to be focusing on band specific antennas so you may consider a collinear array.

A simple, easy and fairly economical one I sincerely recommend is the Cushcraft AX2 tunable from approximately 130 to 170MHz with the 2M Amateur band right in the middle making it a very popular antenna. It comes with an assembly chart so all you have to do is select your center frequency +/- 2MHz and put it together. The "tuning ring" is used to fine tune for the lowest SWR on transmit (impedance matching) which is something you can't do without appropriate licensing but because it makes little difference for receive setting it at the center of it's range will do nicely. You can squeeze a little more gain out of the ARX2B but IMO not enough to justify the extra cost of the radial kit.

I write from experience, I've used them at several locations with excellent results and they're pretty darn rugged, never had one fail. It's got fairly low wind resistance so you can go reasonably high, mine were on 15" foot masts without guys and survived several of our notorious coastal winter gales. New Jersey Transit Rail Operations uses them so what better recommendation do you want?

Now don't expect a tuned band specific antenna to give satisfactory results for general scanning. For that you need a multi-band antenna which by the laws of physics must sacrifice gain for bandwidth. One of those "DC to light" antennas may be sufficient for all round general scanning your local area but in no way can it reach out and touch someone, for that you need GAIN. You may consider two antennas, one for general scanning and another specifically for railroad frequencies and using a coaxial switch to select each one. While no collinear omnidirectional antenna can hold a candle to a Yagi beam for sheer gain do I have to tell you that spinning round and round trying to follow trains is a bit ridiculous? (;->)

If you're thinking of going the dual antenna route let me warn you that trying to use a combiner to use both at the same time can only lead to disaster. Without getting all technical on you "phasing effects" between them cause all sorts of disruptions and leave you wondering why two heads aren't better than one. Do it right with the switch commonly available at any ham radio retail outlet. Just remember you need one for VHF/UHF because using one designed for HF is another disaster in itself.

Sorry about the verbosity, I hope that is all the information you need. (;->) OK guys, that's just the basics so now you can have it with the antenna and coax recommendations sure to follow, don't forget the opinions and the inevitable "I'm a bigger geek than you are" show bound to confuse Einstein. (;->)

 

[Don_Burke]

my radio is the BC785D and as far as i know it doesn't desense. now if it were a radio shack.......

All receivers desense. The question is how much.

Besides the weather stations and even a long distance 2 meter repeater comes in fine so i believe desense isn't the problem.

That _really_ makes it interesting. Is there something in the line that could be acting as a trap?

I'm not sure what to think but even my portable base station with a single dipole doesn't get it to well in that area either. I wonder if the Jpole has just enough gain to get them. maybe i should be a jpole like that for the scanner and have matching sets up there.

A J pole has advantages over a dipole. Higher gain is not one of them.

 

[blueangel-eric]

i forgot to mention the Jpole is on LMR400 and the ST-2 is on the stock RG6. don't know if that makes any difference on VHF.

 

[blueangel-eric]

First of all a mobile antenna can give only so much gain due to limited size. Can the 2 bay dipole idea right off the bat, too large for too little gain and you have the power divider (phasing harness) to consider, too much trouble for too little results. You seem to be focusing on band specific antennas so you may consider a collinear array.

A simple, easy and fairly economical one I sincerely recommend is the Cushcraft AX2 tunable from approximately 130 to 170MHz with the 2M Amateur band right in the middle making it a very popular antenna. It comes with an assembly chart so all you have to do is select your center frequency +/- 2MHz and put it together. The "tuning ring" is used to fine tune for the lowest SWR on transmit (impedance matching) which is something you can't do without appropriate licensing but because it makes little difference for receive setting it at the center of it's range will do nicely. You can squeeze a little more gain out of the ARX2B but IMO not enough to justify the extra cost of the radial kit.

I write from experience, I've used them at several locations with excellent results and they're pretty darn rugged, never had one fail. It's got fairly low wind resistance so you can go reasonably high, mine were on 15" foot masts without guys and survived several of our notorious coastal winter gales. New Jersey Transit Rail Operations uses them so what better recommendation do you want?

Now don't expect a tuned band specific antenna to give satisfactory results for general scanning. For that you need a multi-band antenna which by the laws of physics must sacrifice gain for bandwidth. One of those "DC to light" antennas may be sufficient for all round general scanning your local area but in no way can it reach out and touch someone, for that you need GAIN. You may consider two antennas, one for general scanning and another specifically for railroad frequencies and using a coaxial switch to select each one. While no collinear omnidirectional antenna can hold a candle to a Yagi beam for sheer gain do I have to tell you that spinning round and round trying to follow trains is a bit ridiculous? (;->)

If you're thinking of going the dual antenna route let me warn you that trying to use a combiner to use both at the same time can only lead to disaster. Without getting all technical on you "phasing effects" between them cause all sorts of disruptions and leave you wondering why two heads aren't better than one. Do it right with the switch commonly available at any ham radio retail outlet. Just remember you need one for VHF/UHF because using one designed for HF is another disaster in itself.

Sorry about the verbosity, I hope that is all the information you need. (;->) OK guys, that's just the basics so now you can have it with the antenna and coax recommendations sure to follow, don't forget the opinions and the inevitable "I'm a bigger geek than you are" show bound to confuse Einstein. (;->)

I have a ARX2B in storage but it won't work well on UHF. My broadbanded dipoles works well at UHF as well. I have two phased together now. I just got my phasing harness in the mail. i'm still waiting on connectors to use my LMR400 on it so right now i put it up with old RG59. so far not much different then the scantenna. but with real cable maybe that'd help. i'm not sure the difference between my jpole and the ARX2b as i haven't compared them yet. the old copper cactus jpole i cut up didn't work as good as the ARX2b. It was susposed to have 6db gain but wasn't much better then a 1/4 wave groundplane except it was more work to mount and took up more space. i use my 2m ham radio on the current jpole btw.

I also think the signals from the Hot box detectors are cancelling themself out with reflections from the building to the east or maybe bouncing off the metal storm windows on my house. i have a complete row of windows on this side of the house. the antennas are on the east side of the house. the hotboxes are to the east.

 

[KC9JUM]

Say what you want..... but for most peoples uses the Arrow J poles are plenty good enough...

Most of those Co-linear VHF monsters require a tower. The arrow J Poles are small enough to be put up where needed.. There is always a bigger fish in the world of Antennas...and yes some armchair expert will always come along and suggest a better or bigger one. In the real world the Arrow J poles are fine for most needs. I use one for 220. And it beats most anything else I have tried that did not break the budget. Use the Arrow and forget anything else for most of your casual needs.

Though... a 5/8 wave Mobile NMO on an adapter like the Maxrad MBS adapter will work well. But they are meant to be temporary. I find the water gets into a setup like that easily. I have one for field setups and such. Makes it easy to swap the NMO elements without having to demount the antenna for another.

Those collinear monsters require a tower for Commercial makes. Ham colinears are CHEAP...and just have screw together sections with a wire inside... Diamond antennas are known for bending and shorting in the wind.

Ohh and in response to the bigger is better crowd in here.... Let me show you something that puts a 20foot 6DB collinear to -shame-

http://www.telewave.com/pdf/TWDS-7070.pdf
High Gain and WIDE BAND.. Something a Colinear can never be... the entire VHF-Hi spectrum on one antenna... the caveat? You need a massive tower to support the beast! And a TON of moolah.