Duplexer always needed?

[k8tmk]

I just replaced an old 2-meter antenna with a new 2-meter/70-cm antenna. I currently have only a 2-meter radio connected directly to the antenna. Is a duplexer needed anyway? What concerns me is that the SWR seems to be higher than it should be.

Randy, K8TMK

 

[ffemt601]

I just replaced an old 2-meter antenna with a new 2-meter/70-cm antenna. I currently have only a 2-meter radio connected directly to the antenna. Is a duplexer needed anyway? What concerns me is that the SWR seems to be higher than it should be.

Randy, K8TMK

You would only need a duplexer if :

1. You had one 2 meter antenna and one 70cm antenna and wanted to connect them to a single 2m/70cm radio.

or

2. You had one 2 meter/70cm antenna and wanted to connect it to a one radio for 2m and another radio for 70cm.

How do you have it mounted? What model antenna? What are you using for feedline?

 

[kb2vxa]

For some strange reason dual band mobiles are always a compromise, one band almost always suffers a bit in the SWR department. Tuning doesn't seem to help a whole heck of a lot, get it flat across one band and you louse up the other so the best thing is leave the factory adjustments alone, that's about as good as it gets. Some people are just too picky, as long as it's not so high as to cause too much power fold-back all is well, <2:1 is fairly acceptable.

The above reply answered your question to a T and all I did really is place blame where it belongs, on the shortcomings of the antenna. We're done, you're good to go, no further discussion needed.

 

[k8tmk]

The antenna is a Comet GP-3 base antenna. It is mounted on a mast about 8 feet above a 5-band HF yagi (20-17-15-12-10 meters), and is fed with about 75 feet of RG-8 polyfoam.

The antenna that was previously up there was a G7-144, and used the same chuck of coax. In a wind storm, about half of the G7-144 broke off, and that's why I replaced it. I went with a dual-band antenna, because I thought some day I may use a dual-band radio in the house.

The guy that did the antenna work said the PL-259 connector up there looked fine and there was no sign of any water damage.

There are no adjustments that can be made to the antenna.

Randy

 

[rescuecomm]

Should have stuck it on a piece of masting and checked it before you sent it up. I have a dual band up on my tower and it is not as broad banded as I would have liked. As stated, most multiband antennas require capacitor/coils inside the housing to get a match and this makes the antenna's 1.5 to1 SWR range less than single band units.

Bob

 

[kb0nly]

You would only need a duplexer if :

1. You had one 2 meter antenna and one 70cm antenna and wanted to connect them to a single 2m/70cm radio.

or

2. You had one 2 meter/70cm antenna and wanted to connect it to a one radio for 2m and another radio for 70cm.

How do you have it mounted? What model antenna? What are you using for feedline?

Correction..

You only need a duplexer if your transmitting and receiving simultaneously on the SAME band.

A Diplexer on the other hand does operate as you mentioned.

There is a big difference between a duplexer and a diplexer, but anyway... Sorry just had to toss my two cents in because the term is always used wrong.

 

[ffemt601]

The antenna is a Comet GP-3 base antenna. It is mounted on a mast about 8 feet above a 5-band HF yagi (20-17-15-12-10 meters), and is fed with about 75 feet of RG-8 polyfoam.

The antenna that was previously up there was a G7-144, and used the same chuck of coax. In a wind storm, about half of the G7-144 broke off, and that's why I replaced it. I went with a dual-band antenna, because I thought some day I may use a dual-band radio in the house.

The guy that did the antenna work said the PL-259 connector up there looked fine and there was no sign of any water damage.

There are no adjustments that can be made to the antenna.

Randy

If a wind storm broke a g7 in half I would bet money that there is water in that coax and connector.

 

[mrweather]

Correction..

You only need a duplexer if your transmitting and receiving simultaneously on the SAME band.

A Diplexer on the other hand does operate as you mentioned.

There is a big difference between a duplexer and a diplexer, but anyway... Sorry just had to toss my two cents in because the term is always used wrong.

+1

Using a dual-band antenna on a single-band radio (provided that radio is one of the two bands the antenna was designed for) won't cause any problems. It's not the most ideal but it'll work fine.

 

[ffemt601]

Correction..

You only need a duplexer if your transmitting and receiving simultaneously on the SAME band.

A Diplexer on the other hand does operate as you mentioned.

There is a big difference between a duplexer and a diplexer, but anyway... Sorry just had to toss my two cents in because the term is always used wrong.

Scott,

I am aware of the difference between a duplexer and a diplexer. However, diplexers are most often labeled and sold as duplexers. Since the OP didn't mention a repeater I clearly knew that he wasn't refering to a cavity duplexer. In the end, the answer to his question is the same.

Until you get Comet, MFJ, and many others to correct their products its going to continue to happen.

 

[N5TWB]

If a wind storm broke a g7 in half I would bet money that there is water in that coax and connector.

This ^^^^^ - RG-8 polyfoam would not be my first choice, especially @ UHF. Antennas are a system that include the feedline.

 

[k8tmk]

To answer the possibility of water getting into a broken G7-144 antenna, that is not possible. The G7 is made up of several vertical sections. These sections are connected together via threaded studs. Therefore, a break in any of the sections cannot allow water to get into the feedline (unless it's the bottom section that is broken).

I made some checks last night and noted that a distant repeater (25 miles away) was putting in full scale on the S-meter with an old antenna side-mounted on the tower. The same repeater could be heard okay, but was not even moving the S-meter with the new antenna mounted in the clear above the top of the tower.

Something is obviously wrong!

Randy

 

[kb2vxa]

Not necessarily, a little feedback on what the SWR curves actually look like would be helpful in determining whether it's acceptable or not. Consider this, a meter is not a bridge so transmission line length does affect readings in the shack. For this reason the only way to accurately measure it is at the antenna.

Now as far as reception of that repeater goes I'd say something is definitely wrong but the question is what. It looks like a bit of in depth analysis is called for beyond a simple SWR check. It's time to take things apart and check them all out bit by bit.

 

[k8tmk]

The problem has been solved!

Randy

 

[kb2vxa]

OK, it isn't a state secret or a national security issue so GIVE with it! (;->)

 

[k8tmk]

Well, I found the problem purely by accident.

I just happend to be looking at the old G7-144 that the tower crew took down and noticed it used a type-N connector. The new GP-3 antenna has an SO-239 connector. I had just assumed that the G7-144 used an SO-239 connector.

Therefore, the tower crew just screwed the N connector (on the end of the cable) onto the new antenna without paying any attention to it. This is easy to do, but the center pin of the N connector does not make contact with the SO-239 socket because the hole in the SO-239 socket pin is much larger.

Now, I just need to get somebody to come out and install an N to PL-239 adapter that I bought at our local electronic parts distributor.

Randy

 

[ramal121]

That is one helluva intelligent tower crew...
N connectors will not screw onto a SO-239 without the help of one big monkey wrench.

 

[kb2vxa]

"Therefore, the tower crew just screwed the N connector (on the end of the cable) onto the new antenna without paying any attention to it. This is easy to do, but the center pin of the N connector does not make contact with the SO-239 socket because the hole in the SO-239 socket pin is much larger."

Pardon me if I'm laughing so hard I can barely type! That's EXACTLY what an FCC Field Engineer did while inspecting my CB station years ago and wondered why he couldn't get the jumper to the watt meter to properly mate with the connector on the back of the rig. Oh it gets worse, after pointing it out to him and finally locating the correct jumper he was still wondering why he couldn't get a reading on the old reliable Bird 43. I saw the problem right from the start so I kept my mouth shut expecting more giggles, then he noticed I was tossing a slug in the air and catching it. As he snatched it out of my hand I pointed to the meter with a grin, the deer in the headlights look on his face was priceless as he discovered he was trying to measure 4W @ 27MHz with a slug marked 1,000W @ 450MHz. Now if you want to know the look his trainee assistant had just picture Spock when he arched an eyebrow and said "fascinating".

To make a long story short, after a few letters were exchanged with his boss in New York he was transferred to a desk job in Baltimore. <there isn't a smiley big enough>