One has to be very careful having antennae working in fairly close proximity to each other. I remember back in the '70s whilst in the Navy, we were tied along side, and the Tx/Rx Room was silent, all the equipment was switched off. Another ship had tied along side our ship. I had happened to go in the Tx/Rx room, probably to collect my note book, when I saw a very thin spiral of smoke rising up from a receiver, then vanish. I thought, 'well there goes an RF coil'; some user had forgotten to disconnect the receiver from the antenna after using the equipment. So one has to be very careful. At VHF and UHF, where equipment is operating at defined frequencies and the antennas are mounted on top of a high building, for example. The Radio manager of the site will have a frequency plan (to ensure that none of the user's frequencies interferes with each other), and will state what frequency, size of antenna, etc.... can be used on site. He may also require all antennas to be fed via a cavity filter, to ensure that users do not suffer interference or damage from other users.
I had 2 radios and two antennas close together. When I transmitted 100 watts on one radio, the receive input of the other radio blew out...with smoke. Be very careful. That receive input is looking for micro watts and even milliwatts will destroy it. I think this video underestimates the problem, probably because it happened to me and not him. Disconnect all antennas that you are not using.
This reply should be pinned at the top. Also, from below comments, many folks have asked, "What if I only use (turn on) one radio at a time?" It doesn't make any difference. Feeding to much power into the receiver can destroy small signal semiconductors used in the front of the receiver.
Very interesting discussion. I once measured the SWR on my mobile on 80 meters while driving down a four lane roadway. Beside the roadway were light poles that are 20 to 30m high and about 10m from the antenna. Every time I passed a light pole, the SWR would go up. So, it was obvious that the light poles were interacting with the RF transmission. The poles were in the NEAR FIELD. Thanks for what you do Dave....
You can also hook up a VNA across the two antennas to directly measure the proportion of energy that gets across for different frequencies. I suspect induced currents could cause damage even if the connected radio is for a different band.
This information really helps thank you Dave I have one vertical and a Delta loop I added a alpha delta switch..grounding at the switch and at the antenna...my antennas are VERY VERY CLOSE TO each other using the alpha switch for one antenna at a time while grounding out the coaxle on the 2nd antenna seems to help too with isolation..my backyard is 15 deep by 30 wide
Dave, I wonder if the question was also pointed at "too close to the operator" in which case referring him to evaluation of safe distance from rf would also be advised. Maybe I did not pay close enough attention to the stated question- or just heard the other interpretation of the question. And by the way, your graphic representations DO convey your intent despite acerbic observations hopefully made tongue in cheek. Forge ahead undaunted! Cheers and 73
You might have added that for those of us with small yards there is no point worrying about interactions because you have no choice. It's either interactions or have only one antenna. My spacings are often 5-10 feet. Everything still tunes up OK, and all seems to work well. Don't forget you will be squirting out lets say 100W, and a reduction in radiation on one heading due to interaction must be compensated by an increase in another direction. That 100W has to go somewhere.
thanks for the video...this is exactly the question I have been thinking through the past couple of weeks. I have a Hygain AV-640 vertical and have been considering augmenting that with a hex beam.
I'm wondering if having two hf antennas on a switch makes a lot of rf come down on the other cable. Unless both have a choke at the switch end. Ie the other coax run shield directly picks up the rf from the transmitting antenna.
Hi Dave, just came across your channel and find it incredible helpful so thank you 🙏. I was originally thinking of stacking an Omni directional on the same mast above a beam. I was thinking of a MaCo YQUAD Beam Base Station CB Antenna first and then stacking a Maco V-5000 on the same mast with maybe 10’ of separation and using an A/B switch to change the antenna used. But if I’m understanding correctly 30 to 40’ of separation would be the better way to go. Thanks for your input and help Woody
Would it be ok for a halfwave vertical to have a horizontal yagi underneath it and what effects would the yagi have on that vertical on top and vise versa? Would that yagi acts like a groundplane?
Here's a question, I have a few long wires, 180ft EF, a CF, an OCF, and a few others, however 3 of them cross over and under of one another with about 3 ft between them where they cross. They are hooked to 3 different hf radios, do you see a problem? 90% of the time only 1 radio is on, 10% of the time all 3 are un uses. I do comms for the USCG so at times I may be passing traffic while receving.
I am in an HOA. I already have a VHF/UHF dual band vertical antenna for my ham radio. I am wanting to add a GMRS antenna. Can I put them on the same mast?
I have two Vertical antennas. One is a Tram 1498 CB 18 foot antenna and the other is a 17 foot Tram 1480 2 meter / 70 cm antenna. I plan to put both on one mast and use one at a time with a switch. My question is can I put both antennas on one mast??
Suppose you transmitted/received on 20m on one antenna and with another transceiver you transmitted on 40m on the other antenna. Would that pose an issue for the front ends for either if they both were on? Assuming only one transmitting at a lime.
How can I monitor a CB radio connected to a vertical CB antenna, and transmit on a 100w radio connected to a dipole at the same time? Just keep the space between the antennas separated by x-amount of feet? Thanks for the great videos!
I have v5000 cb groundplane that I use to talk with neighbors who don't have a ham license, but I also use a ocf dipole for 40 and 80 meters, I never use both radios and antennas at the same time, and they are also on a antenna switch to eliminate a signal coming back to either radio. Can I mount the groundplane 5 feet over the ocf on the same tilt over mast without many problems?
How about one radio and 2 horizontal wire antennas? I have one good path to hang wires in the air. I already have an 80-10 EFHW up and have built a 46-60 dipole to use as an NVIS antenna. I haven't ordered the antenna switch yet but it will take in both cables, the dipole will go to a manual tuner first then after to the switch. The rig is a FT 991A. How far apart horizontally could they be?
No definitive distance apart. If you cross them (one N-S and the other E-W, then they can actually cross over each other. If you can, separate them by about 50 feet or so.
I have a cb in my jeep overlander and want to add a dual band. How close is to close for the antennas on a vehicle. Different bands. Only using one at a time
Assuming you mean 144/440 MHz for the dual-band radio, that should not be a problem. It is best to keep the antennas three of more feet apart. A legal CB radio is at 27 MHz and 5 watts. The typical dual-band max power is 50 watts. The interaction is minimal, if any, and they won't damage each other. By the way, the best mobile CB antenna is a 102" stainless steel whip on 6" spring and body ball mount.
I could have ask you the same question. I have the big gap about twenty feet apart from the one I have on a tower.and another question I have is that I haave verticle in the air and the wind bent the antenna at the top. it does not seem to affect the transition will the bend cause me to get out less?
My understanding is that since fan dipoles or antennas like the DX Commander have radiating elements cut for specific bands the interaction is minimal between the elements. The minimal interaction is tuned out during the final tuning. Think of it kind of like a guitar or piano, when is plucked / key depressed the one string vibrates primarily with minimal sympathetic induced vibration in the others as they are not tuned for that frequency. Otherwise we'd never be able to get any intelligible sound out of any sort of string instrument, it would just be a disharmonious mish-mash of sound. Note that my example excludes multi-string chords. Agreed on the multiple of half wavelength separation of antenna's that operate on the same bands. I've watched RU-vids of a couple contest stations talking about their setups for multiple operators and they'll put antennas up 1/2 wavelength multiples apart so that each is in the "null", so to speak, of the wave emitting from the other.
In typical usage, you have one feed line (coax) coming from the antenna connected to one radio. So, you are using one band at a time. The transmit/receive relay in the radio isolates the tx power from the receiver so that the power doesn't damage the delicate small signal semiconductors and capacitors at the front end of the receiver.
Yes. If one antenna is in the near field of another, it will affect the radiation pattern of the radiating antenna in some way. For one thing, that's how you get yagis.
@ke0og, as a follow on question, what if i did want to have 2 radios going at the same time, I was wondering if i could have 2 antennas going and 2 radios so I could say work FT8 while i'm waiting on my turn to get called in a net
I've seen RU-vids of contest stations doing this by having the antenna's for the 2 radios at 1/2 wave multiples separation so that each is effectively in the "null", so to speak, of the other antenna. This may work for differing banks also but I don't know, they'd have to be able to be placed some 1/2 wave multiple of 2 operating bands.
@@tahoma6889 David Casler's example was 2 HF antennas so I had carried on with the HF example. @David Simmons' didn't specify HF or VHF. Due to lowpass filters in the HF radio there probably wouldn't be interference from the VHF radio but I'd imagine that unless the VHF radio has good band pass filter it could be swamped by a powerful close by HF signal. Hmm, time for more research into this... :)
Specifically was thinking about working FT8 while I was waiting around for my turn on the Nightly OMISS nets on 40/20m, made some friends over there been a lot of fun working on my phone only WAS, and you never know when you might get called on
@@lcdmonitor1981 Since the wave length of the 20m band is pretty much 1/2 that of 40m you could separate the antennas by about 20m, you'd have to calculate the real number,. You shouldn't experience much problems, as long as you aren't running full legal power limit. Best to ball park the separation and give it low power test to see what happens before putting too much effort into it. You could also make yourself up high / low pass filters that would further minimize interaction. A last thing I just thought of was having one antenna vertical and the other horizontal could also decrease the interaction. These aren't things I've tried, but information that I have garnered from a wide array of sources of how this type of interactions of antennas can be reduced.
At least he is doing his best and effectively communicating the concepts. If you can do better then don't whine about it, do it. I see from your RU-vid channel there is nothing posted so put up or shut up as the saying goes.
