Spider Antenna - How Tilt of Radials Affects Impedance and SWR


#1

A practical demostration of how the inclination of radials affects VSWR and Impedance

Please see attached photos. Readings of test equipment (RF Vector Analyzer) are:

  1. Spider with horizontal radials
    Z = 26 Ω
    SWR = 1.9

  2. Spider with slanting radials 45 degrees
    Z = 67 Ω
    SWR = 1.3

  3. Spider with vertical radials
    Z = 73 Ω
    SWR = 2.2

Photo 1 of 3 - Horizontal Radials


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Photo 2 of 3 - Slanting Radials, 45 degrees


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Photo 3 of 3 - Vertical Radials


QUICK SPIDER - No Soldering, No Connector
#2

So, if you are going to make a mistake with the radial angles, better be vertically than horizontally.:sweat_smile:


#3

You can’t take too much away from this because the readings are only relevant to this specific antenna, in this specific location, at this very short length. The scope for other variables having an effect, besides the angle of the radials, is high. It would be better to test an installed spider in situ by removing the connector from the SDR and connecting your analyser, and see how adjustments to that spider affect that installation. For more general guidelines this is an interesting site.


#4

When I read the title of the linked page, I felt right at home :grin:


#5

Two points worth keeping in mind:

  • The antenna analyser itself has a 50Ω impedance. As the circuits under test gets further from 50Ω’s, the less accurate the measurement.

  • All the ‘cheap’ RTL dongles with PAL, MCX and F connectors (not to mention the R836 Rafael chip itself) are all 75Ω

My point is only that the chosen receiver should also be considered.


#6

As far as measuring instrument is concerened, the measured VSWR depends on the impedance setting in instrument, which is 50 Ω.

However the measured impedance value Ω does not depend on impedance of instrument.

The measured value in this case for 45 deg tilt is 67Ω, which is between 50Ω and 75Ω. Hence 45 deg slant should work equally good for 50Ω receiver (pro stick’s integral front-end rf-amp), as well as 75Ω receiver (generic dvb-t).

To optimize for 50Ω (pro stick’s integral front-end rf-amp), the radials should be bent down slightly less than 45 degrees.

To optimize for 75Ω (generic dvb-t), the radials should be bent down slightly more than 45 deg.


#7

Interesting that you say that. In 1968 I was taught the very same concept by the chap who elmered me into Amateur Radio. Since then, I’ve seen and heard many hams who have done the same thing for the same reason. i.e to get the feedpoint impedance closer to the transmission line impedance. They’ve done it with HF as well as VHF and UHF antennas. It would seem it’s relevant to more than just abcd’s example.

The biggest factor that affects impedance is height above the ground - in wavelengths.
At 1 GHz, that’s not much distance.

His analyzer is reporting the right impedance numbers for each example.

Bill - KR6K


#8

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Here is a case where the analyzer is connected exactly as described by you.

Please Scroll down to bottom of FIRST POST to see the last 3 photos showing how & where the analyzer is connected.

Franklin-Spider Antenna (for advanced users)

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#9

No need to convince me abcd…

I’m saying your analyzer is displaying good numbers, and the other chap’s assessment that it’s relevant to only your antenna isn’t quite right. i.e. it works for any quarter-wave monopole with a grounplane.


#10

It was not so much to convince you… it was meant to show how impedance seen by receiver varies in a cyclic manner with length of feed coax, and advantage of this behaviour can be taken by trimming the receiver end of feed coax (within last 1/2 wavelength) to improve impedance match & SWR.

Adding a capacitor of appropriate value at a point where impedance 50+jX and X is positive i.e. inductive, can make impedance seen by receiver = 50+j0


#11

I’ve been an Electronics Technician since 1974, and a licensed Amateur Radio operator since 1968.
My current job is maintaining and repairing an ASR-11 Airfield Surveillance Radar and
Monopulse Secondary Surveillance Radar (the ADS-B/Mode-S part of the system) as well
as a WSR-88D Pulsed Doppler Weather Radar.

What you’re describing is line or input impedance. (not to be confused with characteristic impedance)

The input impedance of a line is a function not only of its characteristic impedance, but also of its loading impedance and electrical length (or physical length and frequency).

They are equal when the line is loaded in its characteristic impedance.


#12

Glad to have a qualified and experiend electronics professional in the forum.
:+1: :+1: :+1:

Sure all members will benefit from your contributions to this forum.


#13

Thanks! 'preciate it.


#14

@xlr99 Since you mention height above ground- I have read some interesting articles about “ground” as it relates to RF. Of course some are focused on height as a line-of-sight improvement, I guess my question is whether there is a trade-off with some antennas where increased line-of-sight is compromised by poorer “RF ground” ?

I guess I’m caught up trying to understand the ground as an RF circuit factor vs an element that reflects some portion of the indirectly received signal towards the antenna.


#15

I appreciate your insight into the situation, I was really warning generally about how taking such a short specific test with many variables, and then claiming one variable as a cause for knock-on effects, is somewhat risky. Perhaps it’s not as dire as I made out, but I’m still wary of seeing three tests on stub of coax in one specific situation and then seeing solid conclusions being drawn as to the results seen. Food for thought, certainly, but the electronics engineer in me would like to see a lot more testing on different setups to solidify the results seen and apply them to some theory in order to generalise as to their causes.


#16

I probably should have said height above earth. The antenna still needs the groundplane (radials)
but it’s performance is affected by its hight above terra firma. This is usually more of a concern at HF
because of the wavelength at those frequencies and hence, antenna element length. The rule of
thumb is two wavelengths above the ground (earth) for optimum operation. Since that’s not always
possible, (especially at HF) it usually works out to “higher is better.”

If there enough radials, (the ideal would be infinite, e.g. a sheet of metal)the actual number for optimum performance is frequency dependent) the radials are the RF ground.
Height above the earth affects radiation angle. The effect is much more pronounced, at HF vice VHF and UHF.


#17

And there’s definitely nothing wrong with that.

Not being an engineer, I’m going on 50 years of empirical data.


#18

I started with one of those little 1090Mhz tuned mag mount whips and had it mounted indoors on a windowsill. After a while I moved it to a spider made from a coffee tin with the sides cut and bent at 45 degrees. Performance improved a bit. After a load more coffee I moved to the same tin but simply upturned. Performance rocketed up and gave the best results ever for that whip. Here is the summary with photos. I’d be interested in your thoughts on the results I got for no tin vs tin with 45 degree radials vs vertical tin.


#19

The first tin didn’t have enough metal to make a proper length image element. For optimum results, the groundplane needs to be at least as long as the driven element. Longer is OK. Mag mount antennas perform well on a car roof. (which was suggested by belzybob.

The mag mount antennas included with most of the SDRs aren’t very good. Some of them are downright horrible. The worst part is the feedline. Small diameter coaxial cable, e.g. RG-174 is some of the lossiest stuff made. I noticed the coil of feedline in your picture. As lossy as the stuff is, even a moderate length of it isn’t doing you any favors. True the loss may not be large, but with the DSP the SDRs do, sometimes the difference between receiving a signal and not is only 1 or 2 tenths of a dB. Feedline is to an antenna like a speaker is to a good audio receiver. Even if you have a top-of-the-line receiver, if it’s connected to lousy speakers, it’s still going to sound lousy. Same with the antenna. Connect a good antenna to lousy feedline and your antenna performs lousy. It’s hard to beat the good stuff, e.g. LMR-100 or any of the other LMR coaxial cable series.

I once again tried a metal lid, and I also tried a paint tin but they both reduced performance.

At this point, it sounds as if the metal lid improved the input to the preamp to the point the preamp was overloading your receiver’s front-end, hence the performance drop.

The taller can works because even though its diameter is too small, the vertical length of the can is sufficient to make the image element. Much the way the radials in abcd’s third picture are positioned.
(where the radials are parallel to the feedline)


#20

Here’s a couple of pics of a 4 radial groundplane I made from an N connector and some AWG 12 wire.
I suspend it at the tip of the vertical element with monofilament fishing line, from a rafter in my attic.
Antenna is connected directly to the SDR.
Here’s the stats page: https://flightaware.com/adsb/stats/user/xlr99#stats-76400


(The power supply in the picture was just a convenient way to “hold” the rcvr/ant combo)

I have another just like it outside strapped to my chimney. It’s less than 20 feet above the ground.
Feedline is 50 feet of RG-6. No preamp or filter as I live in a small rural community.
stats page: https://flightaware.com/adsb/stats/user/xlr99#stats-74613

No comparison to the antenna that was included with my RTL-SDR.COM receiver. (v2)
Definitely worth the time to build.