I need some radio reception help, this seemed like the best place to post since it is somewhat technical.  This isn't HAM radio related, but it is antenna related.  I do have my general license, but never built an antenna before.

I have a Dakota Alert 2500 driveway alarm system.  I have one transmitter with 2 receivers.  One of the receivers is on the corner of the house in the garage closest to the transmitter, and the other receiver is in the house on the farthest corner away.  Problem:  the system is unreliable.  I sometimes get people in my driveway without the system triggering (it seems).  I get notified maybe 80% of the time in the house.  The transmitter is about 500 feet away from the corner of the house with heavy woods and hills in between.  Batteries are fine.

To determine the issue, I started out by putting a punch counter on each of the receivers, connected to the output relay of the channel being used.  What I found is that that the receiver in the garage picks up more signals from the transmitter than the one in the house (transmissions that don't register in the house are registered on the receiver in the garage).  What I would like to try is putting a more efficient/better antenna on the receiver in the house to help pick up the signals that are lost between the garage and the interior unit.  This is where I need help.  The unit operates on 433 MHz.

Here are some pics of the unit:







What are my options?  Is the current antenna even worth replacing (maybe it's as good as they come)?  I've looked at all kinds of antennas and am at a loss for how I would actually hook a different one up.  I've been soldering for decades, so I have the equipment and experience to remove the current antenna and solder on something else.  I started looking at dipole antennas, but am confused on how I would connect it to the board (one side of a dipole is the ground/outer shield of the coax, where would I connect that to the board since the current coil antenna has one connection point?).  Can I just used a certain length of wire as an antenna and solder it to the antenna connection on the board after removing the current antenna?  I looked up 1/4 mono antenna as well (mostly referred to as Sander's style antenna, seem to be popular with RC aircraft).  Do I just run a length of wire from the circuit board to the center pin of an SMA connector for one of those?

I'm obviously clueless, regardless of how easy or difficult this may be.  I've been searching for 2 days online and can't find anything related to what I'm trying to do.  Seems like it might be a simple alteration for any kind of receiver, so my Google-fu might just be weak.

Thanks for any information you can provide.

Edit: added frequency of unit, 433 MHz, thanks cjk.

According to the manual your transmitter operates on 433.92mhz. Im too new to the ham world to help you design a better antenna.

Yes, that information is important!  I would have sworn I put that in my original post. I'll update it, thanks.

Ham is not an acronym, it's a slang term for amateur radio.

The frequency your unit operates on (433MHz) is smack in the middle of an amateur band. It's operating under Part 15 rules for unlicensed consumer devices which limit the effective radiated power, hence the permanently attached milquetoast antenna.

Changing your antennas to a full quarter wave instead of the loaded ones that are on there would probably help a little. If the antennas are blocked by terrain from line of sight that's a problem.

Make sure your antenna polarizations are all the same, ie, that all antennas are oriented vertically - or that all are oriented horizontally, and the antennas are broadside to each other.

Attaching a coax cable or connector shouldn't be too much trouble, just have to identify ground on the board (possibly on the underside) to connect the shield to.

If you decide to make antennas, something very simple to make would be a Moxon directional antenna.

No expert, but what I see is a coil that is probably an approximation of the electrical length of a 1/4 wave @ 433-434 mHz, but about the least efficient antenna design I have ever seen. When a wire is coiled like this to serve as an antenna, it represents the proper electrical length, but the radiation efficiency is poor.

Antenna length formulas:
1/4 wave = 246/434 mHz = .557 feet = 6.8"
1/2 wave = 492/434 mHz = 1.13 feet = 13.6"

Were this mine, I'd swap out the 1/4 wave coil for a half wave wire in straight configuration, polarized in the same direction as the transmitter (vertical:vertical, horizontal:horizontal, etc.).

Hopefully someone with more direct experience or better understanding will provide additional info/suggestions.

Absolutely not, you cannot direct connect a 1/2 wave to a low impedance output, you will get nothing.

As mentioned above, polarization is very important for line of sight, ground wave comms. Make sure all Tx and RX antennas are setup with the same polarization, usually vertical or horizontal. Wrong polarization can reduce signal strength by 20+ db ! Check polarization first, before starting to add better antennas.

A long wire is usually a better option than a helical shortened antenna. I would try adding a resonant long wire to the receivers first. This may be a simple but effective solution. A relatively long wire (several feet) connected in series, though a variable capacitor (at the antenna input) may help making tuning easier.

Another option is to try using an external antenna on the transmitter. Coax cable can be soldered to the transmitter. Solder the center conductor to the antenna output and the shield to the "ground" or common wire on the board. Look for a simple but effective antenna sold on Ebay by Ed Fong. You can tell him the center frequency and he will make a custom antenna at no additional charge. I think he sells the antennas for around $20. You may need to build a matching circuit to match the transmitter output to the 50 Ohm coax and the antenna. It's hard to tell the output impedance of the transmitter without knowing the details.

I don't understand why you feel the need to 'build' any antennas, at all.

Most likely, the receiver is suffering from a lack of ground plane. Expanding that will help a great deal.

However, and, as a general class ham, you should remember this, you really need a better transmitting antenna.

Now, I don't know what the law is on that particular unit, and I'm not motivated to refresh myself, but potentially you are violating some regulations tinkering with a permanently-mounted antenna on the transmitter.

I personally would get a 1/4 wave UHF antenna, or even a small yagi, have its' centerpoint adjusted to the tx frequency, and mount that on a pole at the sensor point. Maybe ten or fifteen feet even. Especially if the sensor is on the wrong side of the car (or person) it is detecting. I'd use the most efficient coax I could locate, and I'd make sure the antenna system was resonant to the transmitter.

At distance, and with shit antennas and lots of ground clutter, having the detected object in between the output antenna and the receiver location may be enough to make the system partially nonfunctional.

The center wire, as previously mentioned solders into where the wire rod exits the transmitter. The braided part of the coax should solder to ground, which will probably be on the back side of the board, and may have a conformal coating applied.


I'd test that, first. I'd also ground the antenna pole in case lightning is an issue.

If you're still having trouble, I would then have someone repeatedly trip the sensor, as you walk the receiver around in the space where it lives. Sometimes, just a few feet difference is all it needs - no modifications necessary. Up high, and as previously mentioned, the antennas all need to point in the same direction, preferably vertical (unless you bought the yagi - then it needs to point at the receiver).

Failing that, then I'd either look for a gain UHF walkie antenna, and mod a BNC bulkhead connector onto the receiver, or if I was determined to leave the receiver somewhere, I'd hook me an outside antenna onto the house, and run a coax back to the receiver.

Or, I'd move the receiver to where it reliably picked up the sensor, and then I'd tie something to the alarm output to warn me whereever I was. Example: most doorbells have two dingers, front door and back door. Almost no one utilizes the back door. Tie the output to pulse the back door tone in the door bell.

Lots of stuff you can do there

You sure that's not the center of the ISM band?

Several feet!?! 1/4 wavelength at 433MHz is about 7 inches.

100% certain that the US amateur allocation is 420-450. "Weak signal" (CW/SSB etc) activity on 70cm is at 432.0-432.125, worldwide amateur satellite allocation is 435-438.

See:
http://www.icomamerica.com/en/amateur/amateurtools/2013_BandPlan.pdf

Yes, several feet. Not all antennas are 1/4 Lambda in length. For example, my VHF/UHF antenna is 18.5 ft long and it does have a lot of gain and works very well on 433 Mhz. Obviously he needs to find the length that works well.

Good grief

So what is your gain on 433 with this antenna?

ETA: I am just a HAM ruining no code tech but I thought I had a handle on the antenna stuff.
It appears I am very misinformed.

making an antenna without an antenna analyzer at UHF frequencies will be tough. Very small variations will cause big changes.

Not knowing the impedance of the amplifier output will make that task impossible.

It would probably be a better idea to change the antenna in the receiver to a better antenna since impedance matching is not as important in receivers and see how that works. replace the receiving antenna with a 14 inch vertical wire and see what happens. that is about half wavelength and should do ok without a ground plane

Tram 1481 advertises + 11.7Db @ 440MHz on their 17' vertical, but I dont think this is the best application in this.

Lets be careful and stick to the application at hand, and cite your work.

How about a corner reflector antenna?
https://en.wikipedia.org/wiki/Corner_reflector_antenna

Can you get the transmitter any higher off the ground?

Oh my goodness, what a collection of disconnected factoids! None are incorrect (although large vertical antennas with overall dimensions of much greater than a wavelength do exist, they are typically stacked dipole arrays internally), but I would be very surprised if the OP is not thoroughly confused at this point.

I happen to have some experience with the Dakota Alert MURS product line, and I have had one of their MAPS-BS kits in service for 5 years now, shooting through some trees and ~900ft from my house.

OP:

1. Are you using the infrared or inductive sensor? If you are using the infrared sensor then it is likely you are both missing alerts and getting false alerts. I recommend you change to an inductive type sensor.

2. All of the proper science and engineering principles notwithstanding, have you tried simply un-soldering the existing antenna and throwing a 10ft long piece of wire on the receiver antenna input? It may be all you need to do.

3. Perform good, closed loop testing. Instead of relying on counters, get on the cell phone with someone in a car and have them drive back and forth in front of the sensor and see what happens in the house in real time while you play with the antenna wire. And take the second receiver and put it in the car with the driver--if that receiver does not alarm then you know it is the sensor and not the radio link (see #1 above).

4. If the easy-peasy 10ft wire experiment fails (due to radio link performance, not sensor performance), then "go nuclear" and slap together a 433MHz yagi out of tape measures or coat hangers. Don't get all fancy, just slap one together and see if that works. If it does, then build some nice ones. If it doesn't, then throw in the towel on the 433MHz stuff and buy some of the MURS units.

That's because it's a series of phased elements, not one long hooyah.

Good. Grief.

Is this what the knowledge base on this forum has come to?

Good grief. Obviously you are the only one who knows everything.

we give suggestions based on our previous experience and the knowledge often supported by experience. This is ham radio, not a commercial government outlet where all problems are often resolved by multi-million dollar solutions.

Of course, it would be nice if the OP built a perfect matching circuit and a directional Yagi antenna (some kind of matching will be required if he adds an antenna to the transmitter). The reality is that he wants to extend the range. I have done the long wire solution on the receiver side to a similar receiver and it did improve reception. Was it a perfect solution? No, heck no, but it was easy and functional. Due to wide impedance variations of receivers and the lack of a ground plane, a wire length would be hard to calculate. This is why a "cut and try" solution works best in a situation similar to what he has available.
I never said it was the best solution. Number one thing he should try is to make sure that the transmitter's and receiver's antenna polarization are the same.

Gamma, I think you know I'm a fairly technically proficient individual, at least I hope so! However, many times better is the enemy of good enough. I would be willing to bet there isn't a single person in the forum who has not used a clip lead and some random wire to fix a portable FM radio reception problem in their garage, shop, backyard, etc.

I did, in the same post, recommend a relatively complex, no doubt fully Gamma-knowledge-base-approved method

Yes, but not all antennas are 1/4 wavelength. This is what I was trying to say.
A relatively long wire (long hooyah) as a receiving antenna often works well in applications similar to what OP decribed.  I have done it with garage door openers, weather stations, wireless driveway alerts with great success. Was it the best solution? No, but it was simple, cheap and effective. Obviously for much longer ranges I had to resort to using a dedicated Yagi or Cubic Quads, directional antennas.

Thanks for all the info. Sounds like I have a bit of experimenting to do.

Seriously dude, stop complaining and start helping. If you have a solution for the guy post it, if not explain yourself as to why the other guy is wrong so we can all learn something.

Yup, for receive antennas, just lengthening the stupid thing tends to help alot, more metal in the air, little or no technical knowledge required.

Transmit antennas are another story.

OP how is the line of sight between your antennas, thats the first thing I would consider.