RF Coupling...

[rbritton1201]

I have a Comet H-422 Rotatable Dipole Antenna for use on the 10-40 meter HF amateur radio bands, and I want to install a new DPD Productions "Omni-X" Scanner Antenna mast/antenna on that portion of the Dipole's mast that extends a little beyond where the base of the Comet H-422 Rotatable Dipole base is installed on its mast.

Since the "V" shape of the rotatable dipole is pretty wide as is, I figure that the Omni-X elements should be about 2.5' to 3', or more, from the elements of the rotatable dipole once the Omni-X's mast is clamped onto the extension of the rotatable dipole's mast. I can also extend the length of the end of the rotatable dipole mast further up to create an even a greater distance for the Omni-X elements from the "V" elements of the dipole if necessary. But, I don't know if it would even be necessary with the frequency separation that exists between VHF/UHF/225-900 Mhz. and the HF 10-40 meter bands.

Installation of the DPD Productions "Omni-X" antenna is via a couple of plastic clamps provided so that the manufacturer supplied pre-assembled scanner antenna/mast assembly can be mounted to an aluminum mast.

I am hoping that by affixing the DPD Productions "Omni-X" scanner antenna mast to the aluminum mast that extends up from the rotatable dipole's mount, it won't create RF coupling situation, or that the Dipole elements will block reception via the Omni-X scanner antenna. My calculations say that provided there is at least 1.7 feet of separation from the Dipole elements, there should not technically be any RF coupling occurring.

As an added precaution, I always turn off receivers that may be in close proximity to other antennas when transmitting with my HF, VHF/UHF ham equipment, just to avoid any damage to the scanners that may potentially occur were I to leave the scanner on while transmitting. The Dipole is designed to be rotatable, but I have it stationary, and I'm not intending to use a rotator.

Using the existing mast extension above the rotatable dipole is ideal with respect to grounding that already exists for the Dipole antenna, which is grounded at the base with an 8' copper pipe, then "bonded" to the service ground via #6 solid copper wire. I plan to also install a lightning arrestor at the base of the Dipole's mast to bleed off transients that might build up with respect to the Omni-X antenna. To reiterate, the ground rod at the base of the Dipole is also bonded to the service ground via #6 solid wire.


Comet H-422 Rotatable Dipole Antenna for 10-40 meters, HF transmit and receive.



Aluminum mast extension above the base mounting of the Comet H-422 Rotatable Dipole Antenna (see red arrow).


DPD Productions Omni-X Scanner Antenna

Gain: 3 VHF 4 UHF dBi
Pattern: Omni-directional
Wavelength: 1/2
Elements: 3
Tuned: 118-137, 148-175 & 225-900 MHz
Band: UHF & VHF
Connector: N Female
Cable: RG8X
Cable Length: 16 Inches
Height: 45"
Width: 30"
Weight: 2 lb

Multi Band-Wide Coverage: The uniquely designed OmniX is tuned to multiple bands, giving it wide frequency coverage. Unlike antennas that were originally designed for the ham bands and then marketed as a scanner antenna, the OmniX was specifically designed for scanner radio use and tuned to popular commercial and civil service bands. Elements are specifically tuned for 118-137 MHz, 148-175 MHz and 225-900 MHz, but other bands can be received as well. This antenna can be quite aesthetically pleasing compared to discones and it's also more efficient.
DC Grounded: A metal ground plane is not needed with this design.
Solid Construction: The antenna is constructed of 1 1/2" aluminum tube for the core element and solid 1/4" aluminum round is used for side elements. No flimsy tubing or dimple rivets are used. All aluminum is given a brushed finish.
Isolated Mast: The actual vertical structure is comprised of UV resistant plastic. This allows the actual elements to be raised into 'free space', away from your metal mounting mast.
Weather Resistant: All fasteners are stainless steel and the feed point is protected by shrink tube.
High Quality N Connector: Instead of the usual UHF connector that's included with most antennas, this antenna includes a 50 ohm female N connector, on the end of a small cable pigtail. N connectors are more water resistant and handle high frequencies better. Need help choosing your main cable? Checkout the Coax FAQ.
Includes Mounting Hardware: Price includes mounting clamps to allow attaching the antenna to any standard 1 1/4" to 1 1/2" mounting mast.
Only Partial Assembly Required: The core antenna is completely assembled, along with the feeder cable. The only things that need to be done yourself are attaching the side elements, and putting up a mast.

 

[popnokick]

Your Omni-X and scanner will likely be unaffected when you are simply receiving in the 10-40M ham bands. The problems are likely to begin whenever you key the transmitter in your ham gear, and those effects may range from simple desensing of your scanning receiver, including direct signal injection of your HF transmissions into the the front end of your scanner, right up to the possibility of damage to your scanning receiver due to high RF from your ham transmitter. These effects will all vary depending on frequencies, power levels, and modes used for transmitting in the ham bands.

However, you didn't specify the maximum amount of RF transmit power you use on the 10-40M Amateur Radio bands, nor in what modes (SSB, CW, RTTY, FT8, etc.) Will there be some effect on your scanning receiver when you transmit with the ham transceiver? Quite likely when the scanning receiver is turned on, less so when it is off. Will the scanner be damaged? Hard to say without more information. The effect might be negligible and overcome by simply ensuring the scanner is off when you are transmitting. But the proximity of the two antennas is very likely to produce some effect in the scanner when you are transmitting with the ham gear. You can measure the received RF energy with a wattmeter connected to the Omni-X. It may be very high... or it may be negligible depending on frequency, power, and mode.

If you wish to reduce the effect of the transmitted RF you'll need to separate the antennas vertically and horizontally if possible. The separation would likely need to be by tens of feet if not more.

 

[mmckenna]

I think it's not a good idea. Scanners usually have little in the way of filtering on the front ends. This is necessary due to the wide range of frequencies they cover.
Most scanners go down to 25 or 30MHz, so no filtering down there. If you transmit on 10 meters, you will absolutely overload the front end of the scanner and possibly cause damage.

Even mounting the scanner antenna farther away may not give you enough separation.

You could install an RF limiter on the scanner antenna input. That would keep RF to what should be a safe level, however you are still going to overload the scanner and won't be able to hear anything while transmitting (probably not a big deal…)

As an added precaution, I always turn off receivers that may be in close proximity to other antennas when transmitting with my HF, VHF/UHF ham equipment, just to avoid any damage to the scanners that may potentially occur were I to leave the scanner on while transmitting.

Turning the power off does not necessarily disconnect the antenna from the soft/expensive bits in the radio. It can still be damaged. If you've made it this far without problems, then you probably have enough separation. Still, the RF limiter mentioned above would be a good idea.

 

[rbritton1201]

Well, the Comet Rotatable Dipole is rated at 2,000 watts. But, I had occasion to speak to a tech at Comet. He was honest about it. and sheepishly admitted that I should not transmit over 200 watts, that the coils are just not really designed to withstand much more power for any reasonable length of transmit time. So much for honesty in advertising...So, I'm careful to stay under 200 watts when using the rotatable dipole antenna. After making the purchase of the rotatable dipole, in hindsight, and following an inspection of the coils, it makes sense to me that the coils may not be designed to withstand much more than a couple hundred watts, as they're pretty small compared to most coil antenna coils that boast a 1500 watt rating.

My "go-to" antenna for HF is a Hustler 6-BTV vertical antenna,10-80 meters, and it's rated as 1000 watts. I transmit via an Elecraft KPA-500 amp and KTA-500 tuner, which has a maxiumum output of 500-600 watts. I'm usually on SSB (primarily 20 and 40 meters) or Digital Data (ie: packet type transmissions, FT-8 and JS8Call). The Hustler 6-BTV vertical antenna is probably about 150' from the rotatable Dipole installation, and both of my HF antennas are probably at least 100 feet from the shack. The rotatable dipole antenna is just a backup, and for times when conditions may dictate changing over to the dipole from the vertical, which has not been too often. But there have been the rare occasion where the dipole outperformed the vertical, mostly due to conditions I assume, and that's the reason I erected the rotatable dipole, just as an alternative.

With respect to damage to the SDS200 scanner's front end, I suppose I could put the scanner coax on a switch in the shack, and completely disconnect it from having any theoretical close proximity to the HF antennas when transmitting on them, and also turn the scanner off when transmitting on the HF, VHF, UHF ham bands, etc...But then, there is always the potential that even if I have the coax to the Omni-X isolated via a switch, whether the length of coax itself would be resonate, and thereby affect the front end of a scanner, even if it's turned off.

The VHF/UHF ham radio antenna is in the attic, and I have no problem hitting the repeaters with it, plus I like the fact that it's completely protected from the weather. We just got 9" of snow over the last two days, and having the VHF/UHF ham antenna in the attic provides it with a good measure of "protection" when the weather gets bad, especially for times when I'm participating in ARES activities.

Currently, I have the scanner antenna mounted in the attic as well, and I'm getting decent performance from it. It's a Comet Discone Antenna, 25-1300 Mhz., but that's another embellishment with respect to performance specs published by the factory because everybody knows that Discones are generally not that great above 500 Mhz. I do think that my Discone antenna works pretty well with respect to VHF/UHF., and even with respect to 700-900 Mhz. But, I also think the 700-900 Mhz. range could be improved through the Omni-X antenna. So, I have an eye toward connecting the primarily scanner, the Uniden SDS200, to the Omni-X antenna, and having it elevated "outside," and slightly higher up off ground level than the height the Discone antenna is presently mounted in the attic.

As I mentioned, by utilizing the existing rotatable dipole mast, because the grounding is already at the base of he dipole's mast, setup would be a lot easier were I to mount atop the rotatable dipole's mast. But, I don't want to trade ease of installation for degraded performance due to wave pattern distortions that may be caused by the close proximity of competing antenna elements, especially compared to the decent performance that I'm already getting off of the Discone antenna in the attic.

If after isolating things with respect to the scanner via a coax switch, to mitigate the potential for front end damage to the scanner, then I suppose my concern is whether the approximate three foot separation of the elements on the dipole from the elements on the Omni-X scanner antenna would potentially distort the receptive wave pattern to the extent that the potential for improved reception might be compromised via the scanner, versus the performance I'm already getting off of the Discone in the attic.

No simple answers...

 

[mmckenna]

No simple answers...

There never is.

 

[Ubbe]

Your transmit rig probably have a PTT output, that signals when you push the PTT. Use that to a relay that disconnects the scanner coax or shortcircuit it. It will automatically also work as a mute switch for the scanner so that its audio are not heard in the mic, and you are anyway not listening to scanner audio while you are talking into the rig.

Most scanners have protection diodes directly at the antenna input, before their bandpass filters. They limit to 1,5Vp-p equal to +10dBm. They can't take much power and will either blow open or shortcircuit at higher levels. A cheap HF high pass filter can help that attenuates below 30Mhz and absolutely one of the power limit devices can work, that use the same type of double diode solution as the scanner but hopefully can handle more power. Check their specification of max power and it might not be much different to what the scanner can handle, but probably cheaper to replace than repairing the scanner.

You might need a FM broadcast filter anyhow so why not buy one combined with high pass 30MHz.
HPN-30118 Combined Notch Filter | Scanner Master
LIM-01 RF Power Limiter - VHF/UHF| Scanner Master


/Ubbe

 

[majoco]

When I was an active ham, I had to use a 30MHz low-pass filter in my antenna lead to stop television interference. You could try one of those and also put another one from your scanner antenna lead to ground via a T piece.

 

[popnokick]

High pass, low pass, no pass, mid pass.... let's get this straight. As I understand it the OP would like to ensure that his ham transmitter is transmitting only on the HF ham bands (1.5 - 30 mHz) and ATTENUATE transmitted RF & spurs above 30 mHz so they do not get into his scanner. The definition of that type of filter is exactly as majoco writes: it is a LOW-pass TVI filter.... such as this one -
MFJ MFJ-702B RFI Filters LowPass, MFJ702B
... which should be placed inline in the coax for the rotatable dipole between the ham transmitter and the antenna.
A high-pass filter placed in the rotatable dipole coax would permit RF above 30 mHz into the scanner.... not the desired outcome.