Batboard

Hi folks,

I've noticed that there are a bunch of "all-band" unity-gain antennas out there.

Motorola has the AN000131A01, retailing for about $200, covering 136-174, 380-520, and 698-960 MHz, 22" tall.

PCTEL has an identical-looking PCTWSLMR, retailing for about $100, 22" tall, covering the exact same frequencies. Both appear to be unity gain on all bands, though detailed specs are hard to come by.

I'm curious if the AN000131A01 is the PCTWSLMR produced white-label for Motorola and marked up 100%, or if there is some subtle difference?

Aside from the obvious limited gain, do they work acceptably? I've got a few radios to install in my truck and there's some appeal to running a triplexer with one compact antenna rather than building a porcupine.

I can confirm the Motorola offering is in fact the PCTEL antenna. I can't speak to real life performance, I don't have any experience with them yet.

wavetar wrote:I can confirm the Motorola offering is in fact the PCTEL antenna. I can't speak to real life performance, I don't have any experience with them yet.

Thanks!

To answer my own question about performance... It appears disappointing at first glance. I'm seeing a 2.8:1 SWR at 147 MHz, with pretty much all the useful VHF spectrum being above 2:1. (136 MHz has a dip below 2:1, but it's well above 2:1 by the ham band.) UHF looks good on the 70cm band, but climbs above 2:1 above 460 MHz. I don't have the ability to measure above 480 or so (using an MFJ antenna analyzer).

That said, this is currently with a marginal ground plane. (Using an ammo carton.) Other antennas have worked just fine in this configuration, but it's certainly not a representative test for typical use.

Your MFJ analyzer assumes that the antenna is designed to look always like a 50 ohm resistor on all operating frequencies. That assumption is invalid.

Karl NVW wrote:Your MFJ analyzer assumes that the antenna is designed to look always like a 50 ohm resistor on all operating frequencies. That assumption is invalid.

If you'll indulge my possible ignorance--what should it be expecting if not a 50 Ohm load? Is a radio not going to see the same thing?

The APX radios internally include matching circuits for the antenna mounted on the radio frame. Motorola's portable radios have been designed that way since back in the days of the HT100 and HT200 Handie-Talkie portables.
Let's do a hamshack comparison: Basically you are connecting your 50 ohm analyzer between the output side of the antenna tuner and a multiband antenna array. That segment of transmission line is not going to see a 50 ohm impedance, except if connected to a dummy load Only then will the load side of your analyzer see a 1:1 VSWR, but the dummy load doesn't radiate very well.

Karl NVW wrote:The APX radios internally include matching circuits for the antenna mounted on the radio frame. Motorola's portable radios have been designed that way since back in the days of the HT100 and HT200 Handie-Talkie portables.

Ooh, nifty. I had no idea they included a matching circuit (neither HTs nor the APXs). It makes good sense.

Am I correct in assuming that most non-APX mobile rigs (say, a regular dual-band ham rig, or something like an XPR / ASTRO Spectra mobile) do not include a matching circuit like that? To be honest I've never looked all that closely at a component diagram of them.

My original intent was to run a couple mobile rigs into this antenna with a diplexer/triplexer. I didn't necessarily expect a 1:1 match over the whole spectrum, but I was surprised to find how much of it was worse than 2:1. I didn't expect a matching network to be necessary.

Let's do a hamshack comparison: Basically you are connecting your 50 ohm analyzer between the output side of the antenna tuner and a multiband antenna array. That segment of transmission line is not going to see a 50 ohm impedance, except if connected to a dummy load Only then will the load side of your analyzer see a 1:1 VSWR, but the dummy load doesn't radiate very well.

Thanks, this is a helpful analogy! I appreciate your detailed answer on this.

In a nutshell, I guess I need to buy a multi-band APX!

The other day I wrote:

It appears disappointing at first glance. I'm seeing a 2.8:1 SWR at 147 MHz, with pretty much all the useful VHF spectrum being above 2:1.

...

That said, this is currently with a marginal ground plane. (Using an ammo carton.) Other antennas have worked just fine in this configuration, but it's certainly not a representative test for typical use.

I tried again in a more representative setup with a mag mount on an actual ground plane, not an empty ammo carton, and it's working a lot better. Just wanted to correct my earlier statements since they somewhat suggested it was a poor choice for many uses.

There are lots of peaks and valleys, but I'm seeing 2.1:1 or better from 145 to 160 MHz (and likely higher, as 160 was 1.1:1), and from 420 to 486 (about where my meter cut off). (SWR rapidly climbs above 3:1 below 144 MHz, but that's not an issue for me.) This is with the same MFJ-269 analyzer.

Receive performance on 800 MHz seems good, but that's a pretty meaningless data point.

This is still not a rigorous analysis, but just wanted to correct the record so people happening across this don't draw inaccurate conclusions from my dumb test last time.