RF, metal rods, witches, wizards and stuff

Hi all,

A question for the RF " knowledgeable".

I am placing a transmitter (The RDAS transmitter/antenna to be precise) in the bottom end of an avionics bay. In the bottom compartment there will be two threaded steel rods running adjacent and in parallel to the aforementioned antenna.

The steel rods will be running the full length of the bay past all sorts of other electronic gadgets....past numerous bulkheads, via the M6 toll etc. etc.

I have a couple of concerns:

The first is that the antenna will induce all sorts of electronic noise in the rods and in turn the rods will dissipate unwanted nastiness to other gadgets in the other bays. Is my concern warranted? Can I resolve the issue by shielding the rods? Can I do something electronically magical like connect the rods to the -ve of the battery supplies?

second concern:

Although I am placing the transmitter/antenna some way away, circa 50cm (19") from the GPS receiver I would be far more comfortable if I could shield the rest of the gadgets (and the GPS) in the adjacent bays in some manner. There will be plenty of ply bulkheads in between. Would it be possible to sandwich a layer of tin (OK I am making this up now - but the principle of the question stands! ;) ) in between a couple of bulkheads or perhaps paint the bulkheads with some special "shielding" paint?

I trust that the witches and wizards that are RF knowledgeable can stir up some broth in their respective cauldrons and retort some form of answer as I am afraid I am not skilled in this form of black magic.....

Kind Regards


Reply to
Damian Hall
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Reply to

Yes. And the rods will affect the radiation pattern of nearby antennas, as well. The level of and effect of the induced interference is the question.

Consider that the rods issue exists because the rods are metallic. Shielding will probably involve more metal. By itself, simply shielding the rods is probably not the answer.

Even a grounded rod can act as a parasitic radiator.

The magical things to do would, IMHO, involve shielding the components you are worried about being interfered with.

The real question is whether shielding is even needed.

Does the GPS antenna face "away from" or "towards" the transmit antenna? Are you concerned with data during the recovery phase, ie, does the physical orientation of the antennas or enclosures change upon deployment separation or recovery (up is now down)? You don't want a nearby conductive barrier between the GPS antenna and the "sky".

GPS antennas are very selective and will (should) reject most spurious frequencies.

I guess the ideal circumstance would be a shielded (metal lined) enclosure for the components you want to protect. Be aware that wires (power) outside the shielded enclosure act just like antennas and can deliver RF into the enclosure.

Mounting the transmit antenna away from the rods and other wiring would be the best solution, if possible. Can you extend it opposite to the rod's direction? Can you route the xmit antenna away from the electronic bays via a shielded cable?

If you can move the xmit antenna to prevent any nearby electronics or metal, including wires, from being in parallel with it, you should be okay with typical xmit power levels. You may not even have an interference issue to start with. I would be more concerned with the rods and the xmit antenna radiation patterns and maintaining a constant link with the ground receiver.

If deployment operation depends upon these same electronics, I would certainly do some ground tests first, just in case.

Do you have a diagram or pictures of the installation?

Reply to


You shouldn't have a problem. The GPS will be operating far enough away from any frequency that might cause any resonance with the steel rods and you are not going to be putting out enough power from anywhere else to cause any problems anywhere else.

Your mobile phone still works when it's in your pocket with all the pens and paraphernalia, doesn't it? You don't have to empty your briefcase to ensure a good reception, either.

Unless you are unable to follow your rocket with the aerial, don't worry too much about the polarisation of your aerials either. You could always use a conical horn aerial, particularly if you are going to use 2.4 GHz video. They have an acceptance angle of around 30 degrees, mine does anyway! If you get a weak signal in one plane, twist the tracking aerial through 90 degrees and try again.

Reply to
Bowler's End


try a complete mockup test on the ground to see if there is any interferance? That way, you could see if shielding was actually needed... I assume you are using the RDAS telemetry module, this uses the Linx BBA619-A rf amp, pushing out 50mw. I have tried one of my 50mw beacons, taped to my Magellan Sportrak Color GPS, and had no ill effects.. GPS works on 1575.42 MHz and or 1227.60 MHz, so the harmonics from the TX should be far enough away, but you may get some de-sense in the GPS RX..........

Keep the transmitting antenna as far away from the steel rods to avoid de-tuning and loss of range, or if you are really keen, why not build a small PI network to match the rods into the transmitter instead of using the small supplied one, seeing as you are gonna be in violation of the 433 exemption, by flying 50mw... ;-) A small PI network can be made from 2 small variable capacitors (RS Components 125-660 @ 57p) and a tuning coil with a tunable slug (RS Components 278-8184 @ 95p).....

paul ==============

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Reply to

If the antenna is vertical at 4000 feet, how will you get the signal on the ground ? At least at full power, and not 8-12dbi down ?

The radiation pattern of a 1/4 wave etc vertical antenna is like putting a donut around the antenna wire. now raise the donut up 4000 feet and the majority of the signal is now on the horizon from where the antenna is in the troposphere, not where you are standing with the directional antenna.

now, if you are using a very high gain antenna, you probably are receiving the signal down 8-12 dbi and simply bringing it back up to a level over the noise floor.

if it's horizontal, the donut's power bands are running right into the ground, with the most power.

this link most likely can explain the situation much more accurately then I can.

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I have experienced this phenomena myself.

Reply to

I accept that, but with the limitations that we have we are always going to be playing catch-up unless we start to spend an inordinate amount of money on more sophisticated telemetry set-ups. It's a trade off!

Reply to
Bowler's End

oops... typo slipped into my reply.. The rf amp is a Linx BBA519-A..... Soz for the confusion!


Reply to

The typical method of installing telemetry antennas on "real" rockets and missiles almost always results in antenna pattern nulls at each end of the vehicle. The way to handle this problem is to place telemetry receiving antennas in locations where they are outside of the nulls.

This could be annoying because you will have to back away from the launch site by quite a distance and will not be able to see the launch up close.

As an example of this, one of the best telemetry receiving stations (J-67) at White Sands Missle Range is located off of the range:

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One way to get a reasonable antenna pattern without having sharp pointy things sticking out of the rocket is to use a patch antenna which can be made to wrap around the rocket body. At 433MHz it will be fairly large but the size can be cut down by using a quarter wave patch instead of a half wave. I have the Duroid material to make one and I may have to try my hand at it one of these days.

Reply to
David Schultz

On our systems (2.4 ghz) on the rocket we use a horizontally polarized

1/2 wave dipole and for the receiving station we use home brewed helical antennas. With a helical on the ground you see a 3 db drop with any omni antenna, but as the rocket antenna changes polarization (hoizontal/vertical due to rocket movement especially during parachute deployment) the helical is only down 3db where another linear polarized antenna will go down 20db if the polarity is not aligned. We regularly use over 1 watt transmitters in close proximity to a variety electronic, including GPS and have never had any problems with other electronics being affected just seperate the antennas as far as possible to reduce any eddy currents...
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