Railfan Technology > More antenna questions

Before I start, the concerns a mobile, and yes I have gone through the archives.
Right now I have a Uniden BCT15X in car. Right now I'm using a Diamond NR770HNMO, a ⅝ wave 2m/70cm ham antenna. I thought 144/450 were "close enough" but that doesn't seem to be the case, I was expecting better reception. The scanner itself is ok, I'm mostly listening to local traffic which is a guessing game, and crews seemingly use random channels and at 3 miles I can hardly hear what's going on.
Antenna height generally isn't an issue, but I would like some suggestions. Would it make much difference getting a tuned VHF antenna or is 144 close enough? I would like a dual band antenna to get EOT/DPU, but does that sacrifice VHF reception?

I know mag mounts aren't best, but it's the best solution for me as I plan on selling the vehicle (Explorer); the replacement vehicle will get a permanent install. In the mean time, where is the best place to mount the antenna on an SUV?

Thanks for your help TO.

The antenna cut for 161MHz would outperform one cut for 146MHz.  The question becomes “How much.”  I have a similar antenna on my car and I’m using a mag-mount on the trunk.  If I put it on the roof it bangs the garage door when I pull in or out of the garage.  If I’m going to be away for quite a while I’ll move it to the roof.  Most of what I listen to is local.  Around my neighborhood I can hear the local yard that is about 14 miles away.  I hear the dispatcher of course, and I can hear the locomotives.  I can’t hear anyone on the ground with a handy-talkie.  In the other direction I can hear a detector and trains about 8 miles from me.  My neighborhood is about 150’ above the level of the yard and about the same as the detector.  My home, however, seems to be in a dead spot.  I can hear stuff at the edge of my driveway but loose them 50 feet from there. I mention that because terrain can make a big difference.   I’m a ham and I have the same lack of signal on the ham bands when I am in the driveway. 
 
I have an antenna similar to yours, a Diamond NR770 but with a different mount.  I just did a quick check with an analyzer I have.  One of the things we look at on antennas is the SWR.  The lower, the better.  At 147MHz the antenna had a 1.08:1 ratio.  At 161MHz it was 3.54:1.  It’s a measurement that shows that the antenna has a good match at 147MHz but not so good at 161MHz.  SWR is really a measurement that keeps us hams from burning up our expensive radios when we transit.  Power that is reflected back from the antenna and not radiated by the antenna has to go somewhere and it ends up as heat in the amplifier's expensive transistor.  Another characteristic that can be looked at is the impedance.  The Diamond antenna is supposed to present about 52 Ohms at the designed frequency.  At 147MHz I measured 55 Ohms and at 161MHz it was 22 Ohms.  I don’t have the formula handy but you could look it up but the “Transfer of Power in an electronic circuit” is greatest when the source and the load impedances are equal.  The 55 to 52 Ohm relationship is pretty good but a 22 to 52 is not the greatest.  I also looked at Return Loss at the two frequencies.  I forgot what the values were but there was a real notch at 147MHz and 161MHz was sort of off the chart. 
 
All of this is sort of theory.  How much does it really mean to anyone?  That’s hard to say.  There are so many factors that play into the performance of a Radio System.  The frequency, the elevation of both transmitter and receiver, the terrain, buildings, even vegetation.  If we were talking about a microwave system, things would be more predictable but with the relationship between the train and your car constantly changing, it’s hard to predict results.
 
The Diamond antenna is about 36” from the top of the loading coil to the tip.  If you were going to buy a different antenna anyway, you might experiment with the Diamond first.  I ballparked it and you could probably shorten the antenna by 3 to 4” and it would move the resonant point up to about 161MHz.  That might make a difference.  Remove the Allen set screws at the base.  Tim, K3HX has a great article on how to cut a stainless steel whip safely.  Just search it here on Trainorders.
 
As far as where to mount the antenna on the SUV, I would suggest mounting it in the center of the roof.   It will work just about any place but the center of the roof will make it perform equally in all directions.
 
Hope this is somewhat helpful.  As a ham, I have used several antennas over the past 50 years and generally I would just check the SWR and if that was OK, the antenna was good to go.  There’s other things that can be important also and they all seem to come together in a good way at one frequency.

John

EDIT:  Added hyperlink to K3HK comments on cutting stainless steel antenna safely.



Edited 1 time(s). Last edit at 04/21/16 00:02 by wa4umr.

As John noted, having an antenna cut to the proper length for the frequency range being monitored is important.  Other little things can compromise reception, too.  Is the antenna cable kinked anywhere that would be enough to break the shield in the cable?  Are the connectors properly attached to the cable?  Of all things that can cause bad reception (outside of the radio itself), a bad connector connection to the cable is one of the most common and is often impossible to see.  For the record, I've made many antenna connections and have gotten pretty good at.  That said, I still check every one that I make with a meter to make sure that it's good. 

Then, of course, there is the radio itself.  Great antenna + lousy radio = lousy reception, as I've noted many times before.  No free lunch.  One of my best setups prior to NXDN was a Kenwood amateur TM-261A mobile (predecessor to the great TM-281A radio) connected to a Larsen NMO-150 antenna tuned to the 160-162 mHz frequency range.  With it, I could easily hear a mountaintop railroad repeater 170 air miles from my location.  Narrowbanding cut that range down a bit, down to around 100-120 miles, still not bad.  An easy way for me to check my railroad radio reception is to check it against one of the NOAA weather radio frequencies.  The nearest 3 NOAA stations to me are all 80-100 air miles away.  I can hear all of them with my mobile railroad radios.  By contrast, my very sensitive commercial portable radio with high performance "rubber ducky" antenna will only pick up one, and sporadically at that.

wa4umr Wrote:
-------------------------------------------------------
One of the
> things we look at on antennas is the SWR.  The
> lower, the better.  At 147MHz the antenna had a
> 1.08:1 ratio.  At 161MHz it was 3.54:1.  It’s
> a measurement that shows that the antenna has a
> good match at 147MHz but not so good at 161MHz. 
> SWR is really a measurement that keeps us hams
> from burning up our expensive radios when we
> transit.  Power that is reflected back from the
> antenna and not radiated by the antenna has to go
> somewhere and it ends up as heat in the
> amplifier's expensive transistor.  Another
> characteristic that can be looked at is the
> impedance.  The Diamond antenna is supposed to
> present about 52 Ohms at the designed frequency.

I doubt that the VSWR match has any effect upon receiving.  It is very important for transmitting.  It be willing to bet that a scanner's input impedance isn't anything close to 50 ohms.  It would be interesting to measure how much tuning for resonance affects the antenna gain, which is important for receiving.



Edited 1 time(s). Last edit at 04/21/16 09:04 by radar.

You're asking good questions.  I use a single band Larson antenna for VHF monitoring cut for 161 mHz with an optional whip to screw on for 146 mHz since I'm a ham.  It is mounted on-glass side window back which is one of the worst places for a car antenna, but my SUV offers little alternative for vertical clearance and easy of removal/change is a priority for me.  So I live with the compromise.  It actually works well enough for what I need, but no doubt the signal loss is very significant.  I don't drill holes generally, so the on-glass is good for convenience but not good for reception.

A dual band antenna for 161 and 455 mHz is difficult if not impossible to construct (without manual adjustment when monitoring both VHF and UHF channels in the same scan list) since an odd multiple of the frequency is where antenna resonance occurs.  So 161 mHz on a dual-bander comes out at 483 mHz, quite a distance off frequency for EOT/DPU.  I use a separate radio and mag mount UHF antenna for EOT/DPU when travelling on a train trip to get the needed distant reception.  I seldom need to adjust the UHF radio since there are only a few freqs, so just leave it running on the back seat with a remote speaker so we can hear it ok.  The VHF radio needs more attention while travelling long distances or listening to multiple railroads, however, and I keep that up front.  I find that a VHF antenna is a lousy UHF antenna, in general, but it works for close-in reception and DPUs use a lot more power than EOTs so they are hearable quite a distance away, depending on terrain.

SWR is always hot antenna discussion topic.  Physics says if you have a signal strength loss while transmitting with a given antenna system at a given frequency, you'll have a similar loss in reception as well.  The best course I know is to use the lowest loss feedline (coax or whatever you choose) between receiver and antenna.  Standing wave loss will increase with a higher loss feedline, as far as I know.  Good feedline is more expensive, but hey, the distance is short.  Some will argue that the loss is not significant at VHF and even UHF for a short feedline, and they are right.  However, I do use my mag mount antenna for ATCS 900 mHz reception occasionally, and at 900 mHz the losses are important to control.

All problems I have ever had with mobile installations have been antenna/cable connections.  The coax can take quite a beating in a car if installed under carpets or run along the car frame and eventually kink or crush.  And the connectors do come loose or conductors break inside even if soldered if they are connected and disconnected frequently at the radio.  I had an antenna system quit working in my SUV a few years ago and my buddy told me the Larson antenna was shot.  Shot ? How could that be ? I cut off the old coax connector at the radio end, installed a new one and problem solved.  So I agree with the previous writer, look for connector and cable problems first.  In 30 years of mobile operation, I have never had an antenna (single VHF, dual 2/70, single UHF) go bad so that speaks well for Larson.

The car top is the best place to mount antennas, no question.  Permanent mount will help with the ground plane effect.  With an SUV, the height is problematic since the whip will hit everything at one time or another.  If you must have it up there, make sure it has a good spring in the assembly!

Just about any 2m 5/8 wave antenna will work fine cut to 161 MHz. If the cutting chart does not go that high, you may have to figure it out with a little math. The will the mag mount will work fine too. There is plenty of capacitive coupling through the magnet area as long as there is not too much plastic insulation. Keep the coax cable as short as possible. Coax loss can make more difference than the antenna. 

The only down side is if you are a ham, you cannot transmit on that antenna. I normally have dual band antennas on my cars, but when railfanning, I switch to RR 5/8 wave.

73s, AB6YS

Scott Schifer
Torrance, CA
TinyTrains Website



Edited 1 time(s). Last edit at 04/21/16 22:56 by tinytrains.

Late reply, but the diamond antenna has a serious sharp cutoff outside of the hambands. I tried it due to having a license, and it just sucked - on scanners, ham radios (monitoring outside the bands) and commerical radios.

I have excellent luck with the commerical PCTel and Comtelco antenna's. Although Larsen is very popular, I haven't had the best luck with them for long term use. The PCTel/Comtelco has thicker metal options for the whips - and the PC Tel VHF antenna's are already cut for 19" which gets you in the middle of the band.

Scanners sacriface a lot in performace compared to single band/commerial radios - but the anteanna (and mag mount) are your biggest concerns at the moment.

I am using these right now with good results:

http://www.antenna.com/apg_products.cgi?id_num=10932 <phorum break> <phorum break> FWIW, I am using the same antenna on a Motorola APX7500 and a Uniden 536HP. The Motorola, a commerial radio by design still hears "more" than the Uniden scanner, but both will pick up pretty close (enough) for local railroad chatter and then some. All are NMO mounted with high quailty connectors and NMO antenna kits.

To add a bit more from my experience.  Like other posters, I have had very few mobile antennas fail, but I have had a couple.  Usually the failure occurs in the loading coil at the base of the antenna.  That said, I have Larson NMO-150's that are pushing 20 years old that still work fine.  Cables and cable connectors are almost always the problem point and can be hard to diagnose.  A good SWR meter is the best way to determine if a cable and/or connector has a problem--then it's a matter of isolating where it is in the cable and connectors.  My least favorite job in radio maintenance is attaching connectors to cable.  It's difficult to get done correctly, and almost impossible if one doesn't have the specialized tools to do it. 

Portable radio antennas have their own issues.  The most common point of failure on them is the antenna itself.  A "rubber duckie" antenna can get a break in the antenna internally and it often is impossible to see a defect.  The telling symptom is simply when the antenna quits performing well.  I wear a portable radio on my belt a lot, and I have found that most portable antennas are good for about 1-3 years if they are worn daily.  Another failure point is the antenna connector in the portable radio itself.  This, in my experience, is a much more common failure point in portable scanners than it is in amateur or portable radios.  Again, poor reception is a symptom of a potential bad antenna connection in the radio.  If the radio's reception suddenly gets much worse and changing to a new antenna doesn't remedy it, there's a good chance the antenna connection in the radio has gone bad.  These generally require a trip to a commercial radio shop for repair, and the cost may not be worth it on an inexpensive scanner or amateur radio.

Lots of railfans try to get by on minimal radio setups and, if one is only concerned about hearing local radio traiffic (i.e., less than 10 miles or so away), that may be OK.  However, I look at railfan radio from the perspective of cost vs. benefit.  I figure that the better reception that I have, the less time and money I will waste on "wild goose chases" out in the hinterlands trying to figure out where trains are or are going to be.  The few hundred dollars extra that I spend for a good commerical radio setup over a cheap scanner I figure that I can easily save several times over by the better radio setup allowing me to railfan more cost and time efficiently. At, say, a 40 cents per mile operating cost of a typical car/SUV/pickup, about a 1,000-2,000 miles of driving avoided by having a good radio setup will pay for that radio setup.