"Gary" wrote in message
...
On Aug 23, 4:53 pm, "Peter O. Brackett"
wrote:
Roy:

Thanks for your input on the effects of interference from other
transmitters, especially BC transmitters, point well taken.

I suppose one might add some kind of carefully designed passive filtering
to
the devices to 'notch' out offending BC stations, etc...

But... that would be messy and complicated.

Using a bridge with a tuned detector seems a much better approach. I do
have an old MFJ Rx resistance bridge at hand, but I was looking for a
little
bit better accuracy. I must check e-bay, etc... for prices for a used
bridge of the GR class you have.

My current problem could likly be solved by using one of my (so-called)
VSWR
meters, but I felt I might like to get a little more metrological
capability
for a few bucks.

I've never had a vertical antenna before and currently I'm laying out a
radial field under a new vertical antenna. I just wanted to know when to
stop laying down radials.

I figure that I just need to measure the input impedance of the antenna
Zin
at my frequencies of interest and record the (hopefully decreasing)
impedance values as I lay down more radials. My assumption is that Zin =
Zant + Rg. where Zant includes the reactance and radiation resistance of
the radiator element at my frequency of interest and Rg is the ground
resistance. Hopefully Rg should decrease as I add raials. I intend to quit
adding radials when the impedance decrease becomes 'negligible' ( In the
sense of received S units [smile]).

Thanks again!

-- Pete k1po
-- Indialantic By-the-Sea, FL

"Roy Lewallen" wrote in message

...



I have only one comment which applies to most if not all of the amateur
level products, as well as a very high quality HP impedance meter I used
years ago.

I live about 15 miles from hilltops where the local AM, FM, and TV
broadcast towers are. The signals from those stations are strong enough
to
overwhelm the broad bandwidth detection circuitry in the analyzers (and
HP
vector impedance meter) I've used, resulting in meaningless readings
when
connected to an antenna. I find that I grab my MFJ antenna analyzer for
a
host of jobs like measuring a ferrite core impedance, checking the
length
of a piece of coax, and so forth. But for me it's just about useless for
the job of analyzing actual antennas. When I need to measure antenna
impedance I dust off an old GR bridge and use a portable receiver for
the
detector in order to reject the strong ambient signals.

Of course this isn't a problem for everyone, but it sure is for me and I
don't think my situation is unusual for an urban environment. I've had
to
put common mode chokes on my thermometer thermocouple wire, my light
meter
connecting wire, scope leads, and even in audio circuitry to keep the RF
out. But even one strong local station might be enough to upset a
typical
antenna analyzer.

Roy Lewallen, W7EL

On 8/23/2010 11:15 AM, Peter O. Brackett wrote:
Antenna Newsgroup Denizens:

Apart from the professional 'lab grade' (and undoubtedly very
expensive)
devices made by the likes of Agilent, there are on the amateur market
several antenna analyzers that are more reasonably priced.

I'm currently considering the purchase of such a device, and so...

I'm interested in hearing opinions, pro-con arguments, and/or receiving
pointers to reviews of such devices.

All thoughts and comments will be appreciated, unbiased or not.

Thanks!

-- Pete k1po
-- Indialantic By-the-Sea, FL- Hide quoted text -

- Show quoted text -

Hi Pete, If you are evaluating a radial field for a vertical, then
the value you should be shooting for is 30-35 ohms, with no
reactance. As you know this is 1/2 the impedance of a 1/2 wave
dipole-70 ohms in free space. I have never done this, but an
impedance below 40 ohms with little reactance should provide you with
an efficient vertical. Once you get around this value, I guess there
is nothing to be gained by adding more radials. What your analyzer
tells you, as others have mentioned, depends on the rf in the area.

Gary N4AST

My experience with the MFJ-269 agrees with Roy's -- fine for bench top
measurements but nearly useless for antenna checks if you're close to
high-powered broadcasters. (I'm even closer than he is!)

I have been thinking of an antenna impedance measurement setup using a
HF bridge of some kind, driven by a high-level generator to swamp out
the ambient junk. My 100 wattHF rig followed by a power attenuator
(say, 10db?) would serve to drive the bridge, I would guess.

Anybody have some thoughts on this approach - pro or con? What kind of
bridge would be good?

Larry Coyle, K1QW

In article ,
"Larry Coyle" wrote:

My experience with the MFJ-269 agrees with Roy's -- fine for bench top
measurements but nearly useless for antenna checks if you're close to
high-powered broadcasters. (I'm even closer than he is!)

I have been thinking of an antenna impedance measurement setup using a
HF bridge of some kind, driven by a high-level generator to swamp out
the ambient junk. My 100 wattHF rig followed by a power attenuator
(say, 10db?) would serve to drive the bridge, I would guess.

Anybody have some thoughts on this approach - pro or con? What kind of
bridge would be good?

Larry Coyle, K1QW

The price you pay for living on a Hilltop.... Good for you and every
other RF source......

--
Bruce in alaska
add path after fast to reply

As others have noted, it really is almost a case of you get what you pay for with antenna analyzers. Lesser models are prone to interference especially around high power installations and will give you only pieces of information you may need. One of my personal favorites is the TE-1000 impedance analyzer made by TOMCO which measures pretty much everything you could hope for and has a high amount of protection from interference. I know several AM and even television broadcast guys that use this around towers without issues.

Larry Coyle wrote:

I have been thinking of an antenna impedance measurement setup using a
HF bridge of some kind, driven by a high-level generator to swamp out
the ambient junk. My 100 wattHF rig followed by a power attenuator
(say, 10db?) would serve to drive the bridge, I would guess.

Anybody have some thoughts on this approach - pro or con? What kind of
bridge would be good?


You've got two basic approaches to dealing with strong interference:
1) Make the signal you're measuring huge, so the interference is small..
any detector works
2) Use a tuned narrow band detector.


I would think you could use a simple resistive bridge with option #1,
although the tricky part is getting phase measurements. The current
crop of PC based VNAs and the like rely on having a handy quadradure
source. You could probably use a TAPR VNA, with a linear amplifier on
the Tx port and pads on the Rx port, after calibrating out the amp/pads.

A narrow band detector would be easier. This is sort of how the "noise
bridge" schemes work.. they use your HF receiver as the narrow band
detector. Again, almost any bridge would work for scalar (non-phase)
measurements.

Another approach is the old "three voltmeter meter" technique, which is
essentially a broadband detector and half a bridge, and lends itself to
high power.

The real challenge isn't in making the measurement, but in automating it
so that you can do a "sweep" conveniently. For spot measurements,
almost anything works well enough, but if you want to make 100
measurements it gets real tedious.