Antenna for Dual Band Radio

[Seven-Delta-FortyOne]

I recently got an Alinco DR 638, and I would like to use 2 Larsen commercial antennas for it. I also want to use crossband repeat, so an antenna switch is not feasible.

I'm wondering if you can use a tee, like in the attached image, an split it to 2 separate antennas, tuned for 144 and 440. Will it function like a fan dipole, or is there something about vertical ground plane antennas that would make that not work.

I appreciate anyone's response.


Delta

 

[n5ims]

Not a good idea. Use a VHF/UHF duplexer or dual-band antenna instead. Using just the T will allow your signal (both receive and transmit) to try to use both antennas and cause some really weird results, depending on the two antennas positions relative to the transmitting signal(s). Some good, some bad, some very bad. This is especially important for the 144/440 split since those bands are somewhat harmonically related to start with.

While a dual band antenna is a bit more than a single band one, you'll save money since it generally isn't double the cost and only needs a single mount and feed line (and no duplexer cost too boot!).

Larsen makes a very good line of dual-band antennas that will allow your radio to work quite well, even during the cross-band repeat function. Check out their NMO 2/70 series (Larsen Amateur Mobile Antennas NMO 440B). The "SH" model is their low-profile model, about what a 2 meter quarter wave antenna is. The "B" model has an open coil (similar to the old cell antennas) and the "C" model has an enclosed coil.

 

[Seven-Delta-FortyOne]

Thanks for the response. That is interesting.

I wanted to use 2 antennas for primarily two reasons: 1, I've had very good results with Larsen single band antennas, and 2, as this radio is Part 90, I'll be using it for a little more than just amateur frequencies, and my experience with dual band antennas is that they don't quite have the bandwidth of a single bander.

Also, I have a 75/40/20 fan dipole at the house, with a 1:1 balun, and it works great. Why wouldn't 2 mobiles function the same way?


Delta

 

[mmckenna]

Thanks for the response. That is interesting.

I wanted to use 2 antennas for primarily two reasons: 1, I've had very good results with Larsen single band antennas, and 2, as this radio is Part 90, I'll be using it for a little more than just amateur frequencies, and my experience with dual band antennas is that they don't quite have the bandwidth of a single bander.

Use a single 1/4 wave VHF whip. 1/4 wave antennas are very broad banded, so you'll have no issues covering the 2 meter band well up into the VHF high band. I use a simple 1/4 wave on my truck connected to a Motorola CDM-1550. I've got 2 meter in one zone and my work VHF stuff in another zone. Works just fine and putting the antenna on the analyzer shows less than 2:1 SWR from 144 to well up past 170MHz. Between 144 and 156 my SWR is between about 1.3:1 and 1.5:1.
A VHF 1/4 wave whip is 3/4 wavelength on UHF, so it'll tune up well on parts of the UHF band. I used one with a dual band Kenwood for many years, worked just fine. SWR on UHF was actually better (lower) than on VHF.

Only other way to do this (easily) is to use a dual band antenna, as suggested, or get a diplexer. Keep in mind that the diplexer will introduce some losses into your system. Not a trivial amount, but not so bad that they aren't useable.

Also, I have a 75/40/20 fan dipole at the house, with a 1:1 balun, and it works great. Why wouldn't 2 mobiles function the same way?

Because your fan dipole has a single feed line and one feed point. Two mobile antennas has two separate pieces of coax and antenna feed points to deal with.
If you had a single antenna with two whips, one for 70cm, one for 2 meter, it might work better, but then you just (re)invented the dual band antenna.

There is a way to tune the lengths of coax, but it's not easy to do without doing all the math and then putting it on an analyzer. Dual band, diplexer, or the VHF whip mentioned above will do the job much easier.

 

[Seven-Delta-FortyOne]

Use a single 1/4 wave VHF whip. 1/4 wave antennas are very broad banded, so you'll have no issues covering the 2 meter band well up into the VHF high band. I use a simple 1/4 wave on my truck connected to a Motorola CDM-1550. I've got 2 meter in one zone and my work VHF stuff in another zone. Works just fine and putting the antenna on the analyzer shows less than 2:1 SWR from 144 to well up past 170MHz. Between 144 and 156 my SWR is between about 1.3:1 and 1.5:1.
A VHF 1/4 wave whip is 3/4 wavelength on UHF, so it'll tune up well on parts of the UHF band. I used one with a dual band Kenwood for many years, worked just fine. SWR on UHF was actually better (lower) than on VHF.

That's a great idea. I didn't know that 3/4 wave would be resonant without a tuner.

Because your fan dipole has a single feed line and one feed point. Two mobile antennas has two separate pieces of coax and antenna feed points to deal with.
If you had a single antenna with two whips, one for 70cm, one for 2 meter, it might work better, but then you just (re)invented the dual band antenna.

Good explanation, that makes sense. Thanks for your responses.


Delta

 

[mmckenna]

That's a great idea. I didn't know that 3/4 wave would be resonant without a tuner.

Yeah, it actually works really well.
The 3/4 wavelength will have a funky radiation pattern, but I never had an issue with it. The radiation pattern could be a good argument to use a purpose built dual band antenna, but you'll likely loose some bandwidth on the UHF side.

Larsen, and a few others, also make dual band VHF/UHF antennas designed for the LMR portions of the bands. If you do more Part 90 stuff than amateur, it might be a good compromise.

 

[Seven-Delta-FortyOne]

Well, this sucks.

I have a Larsen single band on the truck. It was originally a 5/8 wave on 2 meter. SWR was dead on 1:1. It also worked great on 6 meters. So I know the antenna and feedline is in good shape.

I took out the whip, and cut it down to 1/4 wave. Now SWR is through the roof, on both VHF and UHF. In fact, it's basically infinity on VHF. The radio shuts down. It's about 5:1 on different portions of the UHF band.

I have no idea what happened, or why. Except that I just ruined a perfectly good Larsen commercial antenna.

:banghead:


Delta

 

[AK9R]

If you had a Larsen NMO150, you had a 5/8 wave antenna for VHF. That it also exhibited a low SWR on 6m was a bonus, but not the design intent of the antenna.

As the cutting chart shows, you can cut that antenna to between 49 and 38 inches for 144 to 174 MHz. I'm assuming that when you say you cut it down to 1/4 wave that your whip is now less than 38 inches. The problem with this approach is that there is a matching coil in the base of the NMO150 and now the coil doesn't match the whip which is why your SWR is wrong.

If you wanted a 1/4 wave antenna, you should have purchased a 1/4 wave antenna. True 1/4 wave antenna do not have a coil in the base. 5/8 wave antennas almost always have a matching coil in the base. Most radio dealers who stock Larsen antennas can get you a replacement whip (W490 chrome, W490B black) for your 5/8 wave antenna so you can get back to where you started.

 

[mmckenna]

Well, this sucks.

I have a Larsen single band on the truck. It was originally a 5/8 wave on 2 meter. SWR was dead on 1:1. It also worked great on 6 meters. So I know the antenna and feedline is in good shape.

Yep, 5/8 wave VHF antennas are base loaded 1/4 wave on VHF low band.

I took out the whip, and cut it down to 1/4 wave. Now SWR is through the roof, on both VHF and UHF. In fact, it's basically infinity on VHF. The radio shuts down. It's about 5:1 on different portions of the UHF band.

I have no idea what happened, or why. Except that I just ruined a perfectly good Larsen commercial antenna.

:banghead:


Delta

W9BU nailed it. The base coil is designed for a 5/8 wave whip. It won't work with a 1/4 wave whip.
You can buy replacement whips pretty cheap. Not sure where you are located but HRO used to sell them, even had them in stock in the store. Pick up a new one and your 5/8 wave VHF/ 1/4 wave low band antenna will be back.
While you are at HRO, get a Larsen NMOQ antenna. It's the 1/4 wave whip. You'll need to trim it according to the included chart in the bag with the antenna. It'll be somewhere around 19 inches. Should cost you $15-$20 bucks.
Or, just go on line and get the cheaper "chrome nut" style 1/4 wave whips. Should be less than $10.00.

 

[Seven-Delta-FortyOne]

Whisky-Nine-Baker-Uniform, and Mr. McKenna, thank you both very much for your assistance.

That's what I did. It is a 5/8 wave VHF, and I cut it to 19 1/4 inches. Rookie move.

I'm in Humboldt county, equal distance form both Portland and Oakland HRO. I order from them online quite often, though.

Thanks again for helping me.


Delta

 

[mm]

How the heck did you get a ham license if you don't know how and what a UHF TEE connector is used for and the difference between a 5/8th wave mount and a 1/4 wave mount?

Maybe you should forget about vhf and uhf HAM FM and get back to real ham roots and concentrate on HF CW and learn some real radio.

The ARRL and incentive licensing is at fault for this idiocracy all in the name of selling more issues of QST magazine and ridiculous RACES/ARES accessories.

 

[AK9R]

Folks, let's try to be supportive and helpful. If you just want to complain that new hams don't know anything, please take it elsewhere.

 

[mmckenna]

How the heck did you get a ham license if you don't know how and what a UHF TEE connector is used for and the difference between a 5/8th wave mount and a 1/4 wave mount?

This isn't called for.
The assumption that simply passing the amateur radio test gives someone in depth knowledge of 2 way radio is, well, cute. The very thought that a 35 question multiple choice test would somehow prepare someone new to the hobby for every conceivable situation is misguided at best. When 7 year old kids can pass the test, or my wife can sit through a 6 hour "Ham Cram" session, your own question about how someone can get licensed shows a clear misunderstanding of what AMATEUR radio is.

Cut the guy some slack.

I can tell you from working in the communications field for 20+ years and hiring a lot of technicians and technician supervisors that I'll hire the person who isn't afraid to ask a silly question every time over they person who assumes they are some level of expert that they aren't, or are incapable of working with their peers in a respectful manner. I'd much rather have a tech who's willing to stop what they are doing and ask a question. That shows a level of responsibility that many people never achieve in a lifetime. That is 1000 times better than the tech that assumes they know everything and make a much bigger mistake.

Maybe you should forget about vhf and uhf HAM FM and get back to real ham roots and concentrate on HF CW and learn some real radio.

And this attitude right here is why I don't use my amateur call sign as my login on this site, or use the amateur radio "badges" on my profile. These sorts of statements about "real ham" don't serve any useful purpose. Trust me, none of us think any worse or better of you because of your "roots". If you think people are judging you based off what modes and frequencies you use, I think you are mistaken.

The ARRL and incentive licensing is at fault for this idiocracy all in the name of selling more issues of QST magazine and ridiculous RACES/ARES accessories.

What's at fault here is AMATEUR radio operators attempting to shame each other. It does nothing for the HOBBY. Notice the capitalized words in the last sentence. Remembering that this is: 1. amateurs and, 2. A hobby, would serve a lot of people well.



@ delta:

Please, keep asking questions, no matter what they are. You asked a valid question that deserved a valid answer. I'll always do my best to answer. If you do not feel comfortable asking a question in an open forum like this, feel free to PM me directly and I'll get back to you as soon as I can.

And please, for the sake of the hobby, ignore the guys that want to claim you aren't a "real ham" because you asked a question or aren't operation on the bands or with the modes they think you should be. I'd bet my next paycheck that MM has asked a few questions in his day. I'm willing to bet there were amateur radio operators that were more than willing to elmer him and help him along the way. I think he just forgot that.

 

[Seven-Delta-FortyOne]

mmckenna, thanks for the help and the support. That was a excellent post, and thank you for the offer of private assistance.

MM, I'm a General Class ham, I got my license by studying. I've probably forgotten 1/2 of what I was tested on, and learned many times more than that actually operating. I consider the license as a license to learn.

I happen to be a California State Licensed General Contractor, along with a Nationally Certified EMT, a Licensed Commercial Driver, and I own a ranch. I have plenty of certificates, and lots of interests and hobbies. Amateur Radio is also a hobby, and a way for me to stay in contact in this extremely rural area. It's not how I define my life's worth. I spend a great deal more time in the California Building Code than I do in ARRL Manuals. Construction is my trade, radio is just a hobby.

As far as giving up VHF FM, and doing HF CW, thanks for the kind offer, but no thanks. I don't know CW, and at this point, I don't have time or desire to learn it. Besides, I like VHF. And I really like HF phone. But don't worry, you'll never hear me calling "CQ", so you don't need to worry about ignoring my call.



Delta

 

[AI7PM]

Somebody needs more bran in their diet. Maybe lock this one now . The man got good answers from adults.

 

[mmckenna]

mmckenna, thanks for the help and the support. That was a excellent post, and thank you for the offer of private assistance.

Always happy to help.

Amateur Radio is also a hobby, and a way for me to stay in contact in this extremely rural area. It's not how I define my life's worth. I spend a great deal more time in the California Building Code than I do in ARRL Manuals. Construction is my trade, radio is just a hobby.

I'm familiar with your neck of the woods, up there in the State of Jefferson. I'm down here along the central coast.
I use amateur radio the same way. The fun of it wore off a long time ago when I started working in the industry. Now it's primarily a tool I use for family communications. I still play around a bit, but after a long day of this stuff and having better things to do with my time, the hobby aspect of it gets pushed aside. Maybe when I retire I'll get more serious about it.

As far as giving up VHF FM, and doing HF CW, thanks for the kind offer, but no thanks. I don't know CW, and at this point, I don't have time or desire to learn it. Besides, I like VHF. And I really like HF phone. But don't worry, you'll never hear me calling "CQ", so you don't need to worry about ignoring my call.

I've learned to favor VHF FM too. Fits my needs well. I've owned HF and 6 meter rigs. 70cm and a few others, too. Over the years I scaled back. HF rig got sold off. 6 meter rig got traded for some welding equipment I needed. 70cm radios sold on e-Bay due to lack of use and needing the room for other stuff. With my work, a commercial VHF radio does much better for my needs. As of today, I no longer own any amateur only radios. Everything I have is Part 90 LMR gear.

Being familiar with your area, I think you'll find that the 1/4 wave VHF will work well. After spending a lot of time in the mountainous portions of the state I switched to all 1/4 wave for my mobile radios. Those out on the flat lands don't understand how mountainous a big portion of the state is. Looking at the public safety, forest service and other users in your area, you'll likely find they've discovered the same thing too. Gain isn't as useful in the mountains as a more suitable radiation pattern is. Your 5/8ths wave antenna still is useful, but I suspect you'll get some better performance out of the quarter waves.

 

[kayn1n32008]

Use a single 1/4 wave VHF whip. 1/4 wave antennas are very broad banded, so you'll have no issues covering the 2 meter band well up into the VHF high band. I use a simple 1/4 wave on my truck connected to a Motorola CDM-1550. I've got 2 meter in one zone and my work VHF stuff in another zone. Works just fine and putting the antenna on the analyzer shows less than 2:1 SWR from 144 to well up past 170MHz. Between 144 and 156 my SWR is between about 1.3:1 and 1.5:1.

A VHF 1/4 wave whip is 3/4 wavelength on UHF, so it'll tune up well on parts of the UHF band. I used one with a dual band Kenwood for many years, worked just fine. SWR on UHF was actually better (lower) than on VHF.



Only other way to do this (easily) is to use a dual band antenna, as suggested, or get a diplexer. Keep in mind that the diplexer will introduce some losses into your system. Not a trivial amount, but not so bad that they aren't useable.







Because your fan dipole has a single feed line and one feed point. Two mobile antennas has two separate pieces of coax and antenna feed points to deal with.

If you had a single antenna with two whips, one for 70cm, one for 2 meter, it might work better, but then you just (re)invented the dual band antenna.



There is a way to tune the lengths of coax, but it's not easy to do without doing all the math and then putting it on an analyzer. Dual band, diplexer, or the VHF whip mentioned above will do the job much easier.

All very excellent advice. I use a Larsen NMO 2/70B and I LOVE it. However, it is attached to a TM-V71a, and is only used for amateur comms, my LMR radio has a Larsen WB150 1/2 wave antenna(I use it on frequencies from 140.xxxx to 173.xxxxMHz) living in Alberta a 1/4 wave does not work as well as the 1/2 wave due to it being pretty flat.

Mmkenna has given you some spot on advice.

 

[gewecke]

Somebody needs more bran in their diet. Maybe lock this one now . The man got good answers from adults.

Good thought. 73, n9zas

 

[mmckenna]

my LMR radio has a Larsen WB150 1/2 wave antenna(I use it on frequencies from 140.xxxx to 173.xxxxMHz) living in Alberta a 1/4 wave does not work as well as the 1/2 wave due to it being pretty flat.

Yeah, good choice for your location. I've got a Laird 1/2 wave mounted on the roll cage of my Polaris Ranger. Due to the lack of suitable ground plane I chose the half wave. I've discovered it works pretty well in the mountainous areas I ride. Having 2.4dB (I think) gain compared to the quarter wave when they have a good ground plane is nothing to sneeze at.
I'm thinking of replacing my 2011 F150, and am considering giving the 1/2 wave a try on that truck. I've noticed the BLM use them out here in the west pretty extensively, but the US Forest Service has stuck with 1/4 wave.

 

[AK9R]

As long as the conversation stays on topic and as long as knowledgeable and helpful people continue to educate rather than denigrate, the thread will stay open.