Confused about diplexer?

[SIMON11]

I have searched about diplexers and I have read a lot of conflicting advice.
I have a discone antenna and a diamond 2m 70 cm antenna. To be quite honest I don't see that much difference in the two. The discone receives marginally better around 33mhz and the Diamond recieves marginally better at 430mhz. I don't see any difference at 140mhz.
I was considering joining them together to get the best of both worlds. I found this device that I thought might be o/k.

http://www.ssejim.co.uk/dip2002ab.htm

But after reading different posts I am not sure. Is it better to just run two downleads and swap them over when required? What happens if I just join the leads with no diplexer. Will this weaken the signal. strengthen the signal or cause interference.

Any help please!

 

[MacombMonitor]

I have searched about diplexers and I have read a lot of conflicting advice.
I have a discone antenna and a diamond 2m 70 cm antenna. To be quite honest I don't see that much difference in the two. The discone receives marginally better around 33mhz and the Diamond recieves marginally better at 430mhz. I don't see any difference at 140mhz.
I was considering joining them together to get the best of both worlds. I found this device that I thought might be o/k.

http://www.ssejim.co.uk/dip2002ab.htm

But after reading different posts I am not sure. Is it better to just run two downleads and swap them over when required? What happens if I just join the leads with no diplexer. Will this weaken the signal. strengthen the signal or cause interference.

Any help please!

If it's not a big inconvenience, without question the best results would be to forget about a diplexer, or any other multi-distribution device, and switch the coax when needed. My only concern with doing that however, would be wear, and tear on the BNC connector on the scanner, if it were done often. Better yet, another excuse to buy another scanner. Then just leave each antenna connected to it's own scanner...problem solved!

Depending on the scanner, it might be cheaper than a diplexer!

 

[SIMON11]

Thanks.
The wear and tear doesn't really matter because I have a BR330T that has an SMA connector but I always keep an SMA - BNC adaptor on and connect my antenna to this. So that my SMA scanner connector never gets worn. I also have a second scanner that I could leave connected to one.
At the moment I only have one coax from the loft and just swapped the antenna over to try them out. Instead of connecting the two antenna in the loft with a diplexer or whatever I can run a second coax from the other antenna.

 

[N_Jay]

As has been said by many before; connecting antennas together can (and typically does) yield unpredictable results.

You may find that some frequencies are improved by up to 3 dB, while others can be ruled by 20dB or more.

There are only two cases when it is fairly easy to tell what is going to happen.
1) Two identical antennas properly phased,
This typically gives you 3 dB gain in the normal pattern of the antenna with a decrease in gain off axis.
e.g. a flattened pattern with omni.
2) two antennas covering separate bands.
This typically gives you the same result as each antenna separately, but may also have a 3 dB loss across the bands.

 

[Al42]

There are very low loss antenna switches. Just plug the antennas into the switch and run a short coax jumper from the switch to the scanner.

Look in ham supply shops for switches. (They'll usually have SO-239 [UHF] connectors, but you may be able to find some with BNC connectors.) Price should be less than a decent diplexer.

 

[joey_corleone82]

OK, here is the problem though. What if you have UHF conventional and 800Mhz trunked in the same bank right? Like for instance I have most of the macomb county michigan UHF freqs in bank 1 but also I have warren michigan trunked system since it is in the same county. If I hit scan on bank 1 and want to monitor it all at the same time with 2 seperate antenna's would the diplexer allow me to do this? with a switch you'd have to be pretty fast to keep up with the scanner hehe

 

[Al42]

A diplexer will feed both antennas to the scanner at the same time. Considering the antennas most people want to run, a single multiband antenna might be just as good. Or you might try it the cheap way first - get a 2-set coupler from Radio Shack for a couple of buck and use it as a combiner - one antenna to each TV outlet and the scanner to the antenna outlet. The loss is higher and the signals will mix but, depending on the local conditions it may do what you need for a lot less money. (Put it at the antenna end and run a single length of good, low-loss cable, and the net result is cheaper than 2 runs of cheaper cable.)

 

[dkostrey]

Another thing to think of ...

Look at the specs for the diplexer. It has a low freq. input and a high freq input (low=110-165mhz, high 220-475mhz). Right off the bat you're killing your 800mhz endevors. Then you would have to find single (narrow band antennas to accomodate the inputs. Not really worth the time and money.

 

[kb2vxa]

Hi Simon and all,

First, read my posts regarding Project Diplexer, my latest may be found in the thread "diplexer help please". That'll give you a pretty good comparison between combiners and diplexers plus a neat little explanation and graphic that'll tell you just what a diplexer does and how it does it.

Now if you choose to use a diplexer you'll have to do your homework and search out manufacturers of CATV diplexers. I found some that will exactly suit your application but sorry, I never bothered to remember where I found them since I was concentrating on one for MY application. The first thing you look for in a diplexer is the frequency range of each port and the crossover point. One for sub band passivity on the low port is what you're looking for, the crossovers range from about 40 to 54MHz and that's all important, you don't want to cut off the high end of the VHF Lo Band.

One very important thing to remember is that diplexers and triplexers made for Amateur Radio applications are just that, they don't work in the frequency ranges you're looking for. While it is possible to Frankenstein something together using a diplexer AND a triplexer the insertion loss mounts rapidly the more crap you put in line.

Alternately you can use a coaxial (not coax) switch to avoid wear and tear on the connectors when you switch antennas. They're available using SO-239 or N female connectors. Make sure of the frequency rating, a coax switch is good up to about 30MHz while a coaxial switch is good up to about 500MHz and is usable up to about 900MHz. They are reasonably priced at any Amateur Radio dealership.

So what's the difference some may ask? A coax switch is simply a rotary switch in a can while a coaxial switch's internal construction mecanically resembles coaxial cable so electrically it resembles it as well.

 

[nighthawks]

Two identical antennas properly phased

As has been said by many before; connecting antennas together can (and typically does) yield unpredictable results.

You may find that some frequencies are improved by up to 3 dB, while others can be ruled by 20dB or more.

There are only two cases when it is fairly easy to tell what is going to happen.
1) Two identical antennas properly phased,

Hello,
I just got these 2 yagi identical antennas, mounted them 1/4 wavelength apart.
I'm using 2 - 50 foot lengths RG6 into my 4 scanners.
With a metal box I made 2 BNC (in) and 4BNC (out) they are all grounded by the box and sodered all center conductors together, works well, no loss.

My question is are these antennas co-phased or do I need to do something different with the coax? I've done much reading but just not sure about co-phasing to identical receive antennas
Thanks John