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AF7V
10-26-2006, 05:39 PM
I'm going to be experimenting with some different antennas for my HT for use both in the field and to improve local communication. To that end I need at an SWR meter that will work at VHF.

Looking on the internet, I have been able to find these three designs:
2.4 GHz Wattmeter and SWR Bridge (http://www.qsl.net/n9zia/wireless/appendixF.html#10)
Wideband SWR meter (http://ludens.cl/Electron/swr/swr.html)
N7VE SWR Bridge (http://www.arsqrp.com/ars/images/backissue_images/1998_images/0998_images/swr_circ.jpg)

Of these, the second is the only one that specifically states it works at VHF. The first though is obviously rated at a higher frequency and the third is used in QRP at HF.

I like the simplicity of the N7VE, but I currently don't have another SWR meter to compare readings against, so I wouldn't know if it was working properly. I also understand that none of these designs would handle very high power (fortunately the HT has a very low power setting, less than a watt).

Has anyone built any of these and tried them at VHF? Is there one design that will work better than another? If so, I'd also really like to know why.

Finally, if the third design would still work well, could a Norcal BLT tuner be used #for VHF (it incorporates the N7VE design, schematic here (http://www.io.com/~n5fc/blt_sch.jpg))?

Bert

WB2WIK
10-26-2006, 06:12 PM
Quote[/b] (ke7hdu @ Oct. 26 2006,10:39)]
>I'm going to be experimenting with some different antennas for my HT for use both in the field and to improve local communication. To that end I need at an SWR meter that will work at VHF.

Looking on the internet, I have been able to find these three designs:
2.4 GHz Wattmeter and SWR Bridge (http://www.qsl.net/n9zia/wireless/appendixF.html#10)
Wideband SWR meter (http://ludens.cl/Electron/swr/swr.html)
N7VE SWR Bridge (http://www.arsqrp.com/ars/images/backissue_images/1998_images/0998_images/swr_circ.jpg)

Of these, the second is the only one that specifically states it works at VHF. The first though is obviously rated at a higher frequency and the third is used in QRP at HF.<

::I wouldn't use any of these.

>I like the simplicity of the N7VE, but I currently don't have another SWR meter to compare readings against, so I wouldn't know if it was working properly. I also understand that none of these designs would handle very high power (fortunately the HT has a very low power setting, less than a watt).

Has anyone built any of these and tried them at VHF? Is there one design that will work better than another? If so, I'd also really like to know why.<

::For VHF use (2m, 135cm, 70cm), a stripline coupler is far better than any other kind. It won't introduce any loss, and it will be more accurate than the types you're considering. Funny part is, considering your specific application, I wouldn't bother homebrewing an SWR bridge at all, because one that's commercially made and works okay is on the market for a "list price" of only $40. It would be impossible for me to homebrew this for $40, the materials cost would exceed that.

Look at this, for example:

http://www.mfjenterprises.com/products.php?prodid=MFJ-812B

Bear in mind if you try to make measurements on "rubber duckie" antennas, or any sort of "whip" that mounts directly on a hand-held (HT), that's very difficult to do, because the way the duckie or whip behaves is very different when mounted on an SWR bridge, as opposed to being mounted directly on the HT. But the SWR bridge referenced here will work fine with conventional antennas, when installed in the transmission line between the transmitter and the antenna.

For $40, I'd just buy it. This particular product uses a valid circuit that actually works properly at VHF; it is a microstrip etched coupler.

WB2WIK/6

KC2GOG
10-26-2006, 06:12 PM
you may want to give this one a go.

http://web.telia.com/~u85920178/use/vswr-00.htm

VK2TIL
10-26-2006, 11:40 PM
Don't buy; build!! You will learn more.

WIK is correct; SWR meters which use toroid-type couplers are good for HF but marginal at higher frequencies. Some particularly bad specimens are barely useable at 28 MHz.

At VHF+, resistive or microstrip types work well; both are simple to build.

The resistive types cannot be left in circuit during normal use of the radio (although I don't list that as a disadvantage, just a comment). The resistors require a power rating similar to that of the transmitter so this type is only suitable for low powers.

One of your samples is of this type but it uses a toroid coupler across the detector arm of the bridge; this will limit the bandwidth. This one also uses a LED as the indicator; whilst simple, this has the disadvantage of insensitivity to low power because of the turn-on voltage of the diode and the step-down effect of the coupler exacerbates this problem.

In other words, this one will require a fair bit of power to operate but will not allow measurement/indication of low SWR; it's a useful indicator but it's not a measuring instrument.

Microstrip couplers are easy to build; you can draw them on the copper stock with a suitable pen. You can also glue copper/brass strips (cut with scissors) to suitable insulated backing.

Old CB-type meters often have well-made troughline couplers; they are often silver-plated and I think they would make good VHF+ couplers. I will try that one day.

(Why they were used in crummy 27 MHz meters is a mystery; they must have been cheaper to make than toroidal couplers).

Microstrip couplers are frequency-sensitive so they will only indicate RF power correctly at one frequency (unless you apply correction as was done in old Oskerblocks). This is not a disadvantage in SWR measurement; you are measuring a ratio between forward & reverse powers and each is subject to the same "error" so the ratio is unchanged.

So, from the ones you have listed, I would comment;

1. Microstrip = good, but this one is small and is best suited to the 2.4 GHz range for which it was built.

2. Bridge-type; good.

3. Bridge-type but uses toroid coupler and LED indicator.

Remember that an SWR meter has two sections; the coupler/detector and the metering unit. The metering units are just a meter(s) and an old CB unit will serve that function; you just have to build a better coupler which can be separate from the meter box.

Commercially- made couplers may be obtained from MiniCircuits. Here is a coupler I made using two MiniCircuits couplers;

http://img95.imageshack.us/img95/3035/photo1zw0.jpg (http://imageshack.us)


All that said, I have some information on a very good design; I won't post it here for reasons of copyright (it was published recently in a magazine) but I have sent you a PM.

KM5FL
10-26-2006, 11:44 PM
My choice? The industry standard, Bird can't be beat. They are pricey tho. There's an ad on QTH.com listing them used at $150.00 including one slug..

KM5FL

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