Dual mount CB antennas?

[md_p97]

I was at a truck stop the other day and noticed all the antenna equipment. I am looking for a good vhf low/high antenna, then spotted the dual mirror mounts, along with the 1/4 & 5/8 wave ssb poles.

So my question is, could I build my own, using the dual mirror mount, with a 1/4 wave on one side, and a 5/8 wave on the other side and expect decent reception on both low and high bands?

This would allow me to connect two antennas to one radio.

Thanks

 

[Concrete1]

There are plenty of dual band antennas that will give you what you need from 1 antenna.
The most common way to do what you are describing (2 diff antennas 1 for each band, 1 radio), is to use a V/U duplexer, such as those sold by Comet Antenna, to combine the 2. This will eliminate the need for special co-phase cables, & SWR/meter retuning of each antenna, which you would need to go thru to try & simply combine the 2 different antennas without a duplexer.

 

[WileECoyote]

What are you planning on hooking to the antenna? If you are only receiving, SWR will not be a factor. SWR can only be checked/adjusted during transmitting, signal going out. His "tuning" would be done by making the antenna the proper wave length for the signal/signals he is wanting to receive. If anyone on here knows of a way to check the SWR of an incoming signal, I sure would like to hear it.

 

[Al42]

I was at a truck stop the other day and noticed all the antenna equipment. I am looking for a good vhf low/high antenna, then spotted the dual mirror mounts

The dual mounts aren't for dual frequency ranges, it's to make the antenna system directional. In return for getting twice the signal fore and aft, you get half the signal to the sides.

along with the 1/4 & 5/8 wave ssb poles.

Antennas don't know about SSB, just about frequency.

So my question is, could I build my own, using the dual mirror mount, with a 1/4 wave on one side, and a 5/8 wave on the other side

Sure.

and expect decent reception on both low and high bands?

Make that "pretty terrible reception on any band". You want "antennas for more than one band" - what has "antennas of more than one type for the same frequency" to do with that?

This would allow me to connect two antennas to one radio.

So would mounting more than one 1/4 wave antenna on a single mount - and it would actually work. Look at the Radio Shack 20-014 for an example.

 

[Al42]

If you are only receiving, SWR will not be a factor. SWR can only be checked/adjusted during transmitting, signal going out.

Is this some new antenna theory? Signals only create standing waves when going up the coax?

If anyone on here knows of a way to check the SWR of an incoming signal, I sure would like to hear it.

You mean other than by using an impedence bridge? Or an antenna analyzer? (BTW, the SWR of that antenna on that feedline is the same whether transmitting or receiving, which is why antenna tuners can be tuned during receive.)

 

[md_p97]

Thanks Al

This is my situation, and what I was thinking...please bear in mind, that yes, I know I can just buy another antenna...that not withstanding...

I live in an area where the local LE still uses the low band....39-40mhz to be exact, as well as some more not too far away, still using 38-46mhz.

When I saw the two antenna's hanging on the wall, and the mirror mounts, I wondered if I could use one antenna of each type (they were different lengths) one on each of the respective mirror mounts, then attatch that too an iron pole outside at a height of about 13 ft to pull in the low band signals...since cb's use this lower band, it just kind of made sense to me.

After a bit of searching, I found a pic of the RS 20-014 and that was the basic idea I had in mind.

Sitting on top of the iron pole, I have a metal flange...which provides a base to hold a rs mag mount....I cut new lengths of antenna from an old cb antenna to upgrade it to the bearmiles post of an improved antenna, but it really didn't help me much.

The flange that is mounted on top of the poles (they are in 5ft lenghts, from home depot, and connected with a joiner for easy manageable removal) has four holes in it, and I am thinking, in addition, I will attach radials to the flange to create my ground plane.

I already have a discone mounted on my roof to listen to milair, but I dedicate it just for that. The second antenna, I use for my handheld and I would like it to be rather portable, so that if I need the mag mount, I simply unscrew the poles from one another, and take it down.

The CB antenna's I saw, I could easily mount to the flange.

Or should I just get a six or eight foot lenth of antenna and replace the whole shebang on the mag mount?

Thanks for your ideas,

 

[loumaag]

You mean other than by using an impedence bridge? Or an antenna analyzer? (BTW, the SWR of that antenna on that feedline is the same whether transmitting or receiving, which is why antenna tuners can be tuned during receive.)

Yes, SWR affects receive.
Yes an impedance bridge is useful for this, but only with a RF generator.
Put an impedance bridge together with an RF Generator and a detector and you have an antenna analyzer.

 

[WileECoyote]

md_p97, heres something that I did with an old 2' fiberglass cb antenna that works better than the rubber duck antenna.

Carefully cut the outter covering off of one of these type antennas.
leave the factory 27mhz wire attached. Get some 14 gage magnet wire. ( you can use larger or smaller, I used 14)
Using one of the many antenna calculators online (run a search on antenna calculator) calculate for the type antenna, 1/4, 1/2, 5/8 or full wave using the top frequency of each band you want to receive.

10-Meter Ham Band 29.00 - 29.7 Mhz
VHF-Lo 29.7 - 50.00 MHz
6-Meter Ham Band 50.00 - 54.00 MHz
Aircraft 108 - 136.975 MHz
Government/Ham 137 - 148 MHz
VHF-Hi 148 - 174 MHz
Ham/Government 406 - 450 MHz
UHF-Lo 450 - 470 MHz
UHF-T (TV) 470 - 512 MHz
UHF 806 - 956 MHz

The calculator will tell you how long each wire (antenna) will need to be for each band/frequency. measure and cut each length as specified by the calculator.

solder one end of each wire to the base of the antenna where the 27 mhz wire is already attached and wind the wires up the antenna and tape in place at the other end with electrical tape.

Once all the wires are in place, wrap electrical tape up the entire length of the antenna to make it look like it did before cutting the original covering off.

Mount the antenna where desired and run coax from it to radio. It should work, mine did and way better than the stock rubber duck antenna did for mobile use.
I can pick up 7 surrounding countys, some over 50 miles out.

Just an idea. You may not want to buy a new antenna to do this with. I found the one I used on the side of the road. But you could probably find one at a yard sale or flea market for cheap, or you may have one laying around..

 

[md_p97]

Thanks Wiley.....I might just give that a try....I guess what I am trying to do is build my own J-Pole....I like the design on the old RS 20-014 and it got pretty good reviews on the Low band.

Ultimately, I plan on getting a digital later this summer and will probably buy a pre made antenna...but for now, I'm enjoying experimenting.

 

[Al42]

I guess what I am trying to do is build my own J-Pole

The J-Pole is a single frequency antenna. The short part is part of a parallel line that's used as a matching section - it's not part of the antenna.

 

[Al42]

md_p97, heres something that I did with an old 2' fiberglass cb antenna that works better than the rubber duck antenna.

Carefully cut the outter covering off of one of these type antennas.
leave the factory 27mhz wire attached. Get some 14 gage magnet wire. ( you can use larger or smaller, I used 14)
Using one of the many antenna calculators online (run a search on antenna calculator) calculate for the type antenna, 1/4, 1/2, 5/8 or full wave using the top frequency of each band you want to receive.

1/4 wave. 1/2 wave and full wave would present almost infinite impedence at the end, not the 50 ohms you want. A 5/8 wave antenna would need something to match it also - it's not 50 ohms.

 

[WileECoyote]

1/4 wave. 1/2 wave and full wave would present almost infinite impedence at the end, not the 50 ohms you want. A 5/8 wave antenna would need something to match it also - it's not 50 ohms.

I used the calculations for a full wave and it works pretty good. Considering I can now pick up a hell of a lot more than the duck antenna was producing with some of the agencies being more than 50 miles away. Maybe I'll redo another one I have using 1/4 wave calculations and see what that does. Maybe I will be able to pick up transmissions a 100 miles out. What do you think :?:

 

[Al42]

1/4 wave. 1/2 wave and full wave would present almost infinite impedence at the end, not the 50 ohms you want. A 5/8 wave antenna would need something to match it also - it's not 50 ohms.

I used the calculations for a full wave and it works pretty good. Considering I can now pick up a hell of a lot more than the duck antenna was producing with some of the agencies being more than 50 miles away.

You're telling us that an external random-length wire works better than an internal rubber duck - I can't argue with that, since I know it's true. The only antenna that doesn't work better than an internal rubber duck is a shielded dummy load.

Maybe I'll redo another one I have using 1/4 wave calculations and see what that does. Maybe I will be able to pick up transmissions a 100 miles out. What do you think :?:

Since I never connected coax to the end of a full-wave vertical, and I have no idea what length coax you used, so I couldn't even model the installation, I have no idea what will happen.

 

[WileECoyote]

You're telling us that an external random-length wire works better than an internal rubber duck - I can't argue with that, since I know it's true. The only antenna that doesn't work better than an internal rubber duck is a shielded dummy load.

Who said it was random length :?: , each wire was cut to full wave length, upper most frequency of each band. Whats an internal rubber duck? All the ones I have ever seen were mounted on the outside of the radio.

Since I never connected coax to the end of a full-wave vertical, and I have no idea what length coax you used, so I couldn't even model the installation, I have no idea what will happen.

I'm starting to wonder if youv'e ever hooked a coax to anything.