Pacific Crest Antenna

I think we bought a Pac Crest antenna not long after we got our system. We have Hipers with a Pac Crest 35w radio. The antenna that came with it was to short for the best range, so the one we bought was longer. I looked at a yagi, but I didn't think we would be happy with it being so directional, and it looked like it was going to be a few hundred dollars to get one. I would think with a 4 watt radio, your range is going to be limited compared to some of the higher wattage radios.

The Pac Crest radio (model PDL4535) I use with my 4700 base & R8 Rover works like a champ. It has high/low settings to help with battery. Low setting is good for local jobs (a mile or so), high setting will get me 4 or 5 miles.

I've got 10 or so frequencies I can switch to if needed. Just a couple button pushes, but can be a PIA if I'm a couple miles away and somebody is stepping on me.

hope this helps in your decision making & good luck

I'll just add, the length of the antenna whip is usually based on the frequency your operating on. It looks like the radios you got are on a different frequency than the 35 watt style radios, so they make take a different whip length.

Thanks for the replies -- the ADL Vantage (non Pro) operates on 430-470 but most antennas are either 430-450 or 450-470

Looks like I'll possibly grab a smaller unity gain antenna and also grab a nice ~5db good quality build (laird/etc) antenna.

We had very good results with the Laird (formerly Antenex) B4505CN. 450-470-MHz, 5 dBi gain.

Note the "N" at the end of the model designation - that stands for "No ground plane" - the situation you have when the antenna is mounted on top of a mast. Extremely important!!

When mounted on the roof of a vehicle the metal roof serves as a ground plane for the antenna. Since this is a common configuration, the antennas your local two-way-radio dealer is likely to have in stock are ones designed for use with a ground plane. Don't buy one of those for use on top of a mast.

IMHO you would be better off just buying the Laird B4505CN and forgetting about the unity-gain antenna. The reason being that unless your crew is the most mindful crew in the world eventually they will end up with the 5-dBi rod and ferrule on the 0-dBi load coil and vice-versa and nothing will work right then.

GB

GB --

Thanks a bunch for that reply -- definitely what I was looking for.

I am very familiar with the Laird antennas.. we use them in our offshore environment -- but most of them are the fiberglass models.

I have not used the B4505CN yet so that is good feedback.

I'll second Glenn's comments. I use that setup and it works very well.

I've never heard of using directional antenna's for GPS RTK but must say I'd wondered about them.

My large (3'±) base antenna is excellent for reaching up to 6-8 miles when turned to high. I put it on a 4.5 metre painters pole and get it up in the air when I'm wanting to wander far away from the base.
Never used the short antennas for my Base though, only on Rover (PDL)

The B4505CN comes untuned.
Apparently the tuning is critical in gaining optimum performance/range. The package I have indicates that for 450Mhz, the wire rod length should be 10", for 460Mhz the rod length should be 9 3/16" and for 470Mhz the length should be 8 9/16". Lengths for frequencies between those mentioned can be interpolated. Your antenna should be tuned to the length stated on your FCC license and then hope that nobody else is there!

As far as where to measure rod length (total length or exposed length) I never received a definitive answer from anyone. Which is why I don't use the B4505CN.. a 12" Gainflex has been serving me well.

Tuning radio antennas

Tuning radio antennas by cutting the antenna rod to some chart-specified length always seemed overly crude to me, especially after the time back in 1998 when we encountered a situation where the manufacturer's documentation called for a length longer than the brand-new antenna rod. Duh!

That same month I purchased a Bird AT-400 antenna tester, one of the best investments of my GPS-dealer career.

This device measures antenna response across a range of frequencies, making a sweep through the user-specified frequency band about once a second. It plots out a trace of the antenna response (usually Voltage Standing Wave Ratio - VSWR) against frequency. The graphical information provided makes it very to test radio-antenna circuits and tune radio antennas. Sure beats the old routine of swapping out parts to try to isolate the problem.

The Bird AT-400 is now discontinued; the current product is the Bird AT-500 Antenna Tester. The AT-500 sells for around $1400, so the total outlay including a few adapters for various coaxial connectors would be around $1500. IMHO every RTK-GPS-equipment dealer should have one of these testers or something similar, but I doubt if any do.

My reading of Dave's post above leads me to believe that he is concerned that a tuned radio antenna will confine the user to such a narrow frequency band that changing from one frequency to another will render the antenna useless. Our experience, backed by measurements, show this is not a problem in most cases.

The image below is a VSWR trace (captured on the AT-400 then downloaded to PC) from a fresh-from-the-bag (untrimmed) Laird B4505CN antenna across the frequency band from 450 to 480 MHz. Looking past the "noise" that is picked up in the trace, the antenna shows a VSWR of 1.0 (the best VSWR possible - as good as it gets) from about 455 MHz to around 465 MHz. Most private-sector RTK GPS users are issued a set of 13 frequencies ranging from 461.025 MHz to 464.750 MHz, so this antenna will perform very well across the range of their frequencies. Across the 450-470-MHz frequency band the maximum VSWR is about 1.025, which is not bad by any means.

This particular antenna could be optimized for the 461.025-464.750-MHz band by shortening the antenna rod a little and moving the best-response range up in frequency a bit, but for all practical purposes it is fine as is.

Note that this trace is a snapshot of one specific sweep - if one is watching the actual updates on the AT-400/500 screen it is easier to see what is noise (spikes that appear in one second-by-second trace and not the next trace) and where the best-fit curve would lie.

GB

Tuning radio antennas

GB --

Wow, great post.. thanks!

Now I know who to go to for my antenna specific questions/tuning/testing =]

And thanks for that antenna tester link.. one of the companies I work with should buy one of those.