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Railfan Technology > Using one antenna ,with two radios,can it be done?

Date: 03/25/10 18:20
Using one antenna ,with two radios,can it be done?
Author: SCL985

Have a Yaesu VX-170 and a Yaesu FT-270R. Can both radios be connected to

one antenna. If so will be issues of signal loss,or other problems, with this

configuration ?


Date: 03/25/10 19:36
Re: Using one antenna ,with two radios,can it be done?
Author: hdralleiii

Are you inquiring about a base or mobile antenna? The answer to both is yes BUT it will require a quality splitter with isolated outputs to function correctly. Some people simply use 75-ohm TV splitters and they will work but not ideally and actually the radios can interact with each other causing undesireable results if you are not careful.

I have used a 75-ohm model with decent results but the 50-ohm guys are really what you need. One downside is they can get pricey (more than both radios combined) and generally need to be powered units for best results (my personal experience)

I'd like to hear more info about what you are trying to accomplish and if it is in a vehicle or not.


Date: 03/25/10 20:05
Re: Using one antenna ,with two radios,can it be done?
Author: QU25C

I have to ask why you need two on at the same time? and two antennas will be cheaper but then placement will change what you pick up.

Date: 03/25/10 21:06
Re: Using one antenna ,with two radios,can it be done?
Author: Wizard

If you're just wanting to listen, buy a SO-239 tee splitter and feed both radio at the same time with a SMA-to-PL-239 cable unless of course the antenna lead-in cable end is a BNC in which a BNC male tee combined with a BNC-to-SMA adapter cable will be needed. But you can always find a BNC-to-SO-239 adapter cable, a male PL-239 union coupled with a SMA/SO-239 adapter piece & then presto ! I've run to mobiles the same way for listening only. Transmitting ? No way as it's one or the other but not at the same time. And I haven't found any line losses that affects the radios.


Date: 03/26/10 06:33
Re: Using one antenna ,with two radios,can it be done?
Author: K3HX


In a perfect world, half of the power from the antenna would go to each radio for a loss of signal of about 3dB as seen by each radio.

75 ohm splitters as used for cable TV systems are really pretty awful for this application. Many of them are "resistive splitters" which are very lossy but are used for cable applications as they are less likely to send reflections back down the system if a port is left unterminated. These reflections can cause "ghost images" on the TV. On the other hand, the lossy resistive nature of these splitters can "mask" the impedance mismatch between the input and output.

If you need to run 2 radios from one antenna, the proper way is to use a "multicoupler" which has been mentioned by another poster. A good quality device of this sort can easily run into hundreds of dollars.

Another way to solve this situation is using "linear transformer sections." If you have access to instruments or know of a ham operator who has an antenna analyzer or even a "dip meter" you can cook up a way to divide the signal with much less loss than either the CATV splitter or just using a "TEE" connector.

There are several concerns with this technique.

1....You need to know how to solder PL-259 connectors using lead-tin solder.

2....It will work over a limited range of frequencies. Thus, if you set it up to operate on @161 MHz, it will have significant losses on 2 meters (@146 MHz.) It will likely work fine from RR through NOAA frequencies, however.

3.....Instruments are needed to trim and tune the device for proper operation.
3a....If you're willing to give up some performance, you can do without the instruments and just carefully measure and cut the transformer sections. See step 5 below.

4.....You will need to buy 75 ohm RG-59 coax with copper braid and special RG-59 adaptor sleeves to allow the larger diameter of this cable (compared to RG-58) to work in a PL-259 plug.

5....After all this is done, the likely result is better reception of weak signals. I'd try the
simple "TEE" scheme as proposed by "Wizard" which has serious impedance mismatch problems and see how it works for you. In urban areas, this simple "TEE" method may increase susceptibility to overloading and interference from strong stations. If you are not satisfied or just wish to have the setup "right." Contact me and I'll do the math for the kind of RG-59 you were able to secure, giving you the suggested lengths and construction instructions.

Be Well,

Tim Colbert K3HX

Date: 03/26/10 09:34
Re: Using one antenna ,with two radios,can it be done?
Author: LVFoamer

I always use two radios. The FT 270 on the railcom roof mount and the FT 250 with the diamond for when I get out of the car. I will never go back to one radio again.

Date: 03/26/10 10:04
Re: Using one antenna ,with two radios,can it be done?
Author: Wizard

K3HX is more experienced than myself, but from my own personal experiences using a simple signal SO-239 splitter "tee" like I currently have in my vehicle, thus far, I'm not having any issues, front end overloading or performance deficiencies. With my 2 Yaesu radios (2 & 70 meter) although not the two radios in this radio but of the same brand, I can sit up in the Donner mountain range and listen to two separate conversations between the DS and the crews without any problems. Maybe why I'm not having any problems stems from how I installed everything is reason why I can listen to what I want when I want.

And yes, one has to be able to solder together those Pl-239 connectors. Perhaps the approach I took is one reason why I get great reception despite using a splitter. All I'm using in my mobile ham shack is the Larsen NMO cable kits with their RG-58A/U cable and how each cable kit was installed since every one of them (4 kits) were installed permanently underneath the vehicle's headliner in such a way so that any losses are very minimal. And using 2 Daiwa switches anchored to the vehicle's tranny hump probably bodes well, too.

My antenna's are cut where they should be and I should note that my Railcom RR tuned antenna despite my cutting it, works pretty well on 2 meters that I found out by accident. All of the antenna setscrews are set with medium strength Thread-Loc so they don't loosen up & are sealed against any possible moisture intrusion including the point where the whip enters the base and around the base itself with a caulking sealant I use in my line of work.

And the radio power ? All of it put in nicely & well protected !

Next project: Mounting my just bought ATAS-120A antenna........

But I have found that those Radio shack BNC splitters aren't exactly the best. Why ? The BNC connections in the past were never "tight" unlike the "tightness" I get from my Diamond SMAP-BNCJ adapter for my VX-6R. All of my PL connectors are a firm tight connection.

Case in point: Listening to the 2-way conversation between the DS and crew while listening to another 2-way conversation between the DS and crew out on the East Valley line up around Wheatland while I'm parked up in the Baxter area down in a valley with plenty of high imposing hills surrounding my vehicle.

A lot of how good of the reception one desires stems from one's personal approach in installing their antenna's & radio power. Go about it the cheap & inexpensive way with magnetic mounts & cigarette power adapters OR install the antenna cable kits & power knowing that the end result will give one the best performance possible. At least I know that when I key up the mic, my signal will get out no matter if I'm in downtown SF in heavy traffic surrounded by a lot of buildings rag chewing to a fellow ham in Vacaville some 50 miles away or just sitting around here (Fairfield) and rag chewing with another ham up in the Auburn area which is about 75 miles away.

Or listening to a 2-way talk between the DS & crew way up in Roseville when I was down close to Benicia and listening to another 2-way talk between the DS & crew clear out in Tracy from the same spot in Benicia.

No Radio Shack or Uniden scanner coupled with magnetic antenna mounts will ever give this kind of performance !!!

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