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Spacing between vertical antennas
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by N9ZWY on June 10, 2001
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For Field Day this year, we will be at a site that has extremely limited space...perhaps 20' X 20' or so.
Our goal is to operate with two HF rigs this year, which entails 2 HF antennas. I currently have 2 vertical HF antennas, one being a GAP Challenger, and the other is a Hustler 4-BTV. Both operate well with radials in place.
Where we are at is this: we've never operated two HF verticals close to one another before. Having been in and out of ham radio for the past number of years, I'm afraid some of my learning has been lost. I'm simply not sure of hazards that may arise when I place two antennas relatively close together.
The question is this: other than the obvious dilemma of operating (transmitting) two radios on the same band, that is near in frequency to one another, is there anything that we should really worry about? Specifically, if the goal is to run two radios simultaneously at the same time on different bands, are there any preventative measures that I should consider now before the event.
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RE: Spacing between vertical antennas
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by N2MG on June 11, 2001
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I hate to be gloom and doom, but in that small a space you run the real danger of not only overloading the front end of one radio with the RF of the other (making RX very difficult if not impossible), but of seriously damaging same. It doesn't matter if you try to operate on the same band or not. If one antenna is being driven by 100W of 7MHz RF and radiating a few feet from another antenna - that other antenna will bring a very large signal (watts?) down to the connected receiver regardless of whether this receiver is on 40meters. This could damage the receiver - even if the radio is OFF.
What to do:
1. Try to use a horizontal (dipole) antenna as the second antenna (as opposed to a vertical) in an effort to gain some polarization-based isolation. However, I suspect that at <20 feet spacing you'll still have lots of coupling (trouble).
2. Try using single-band bandpass filters inserted into the receive path of both radios, selected, obviously for the radio's current band of operation.
3. Try to use single-band antennas; antennas that are non-resonant might help because they "reject" out-of-band signals.
4. Last resort: Don't try setting up two stations in that area.
73 Mike N2MG
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RE: Spacing between vertical antennas
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by WB2WIK on June 13, 2001
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Hmmm...
...I don't agree with the doom & gloom posting, regarding potential for RX damage.
I've operated multiop stations for years, running 1500W output power to beam antennas that were stacked on the same tower, often only a few feet apart, with one beam transmitting and another beam receiving on the same tower, at the same time, and have experienced zero problems. In some cases, the driven element to driven element spacing has been just several feet, and the bands used have been very close in frequency, e.g., 14 and 21 MHz, or -- egads! -- 7 and 21 MHz, which are harmonically related, so the 7 MHz beam is quite an efficient radiator of 21 MHz energy.
So, I really don't see that particular issue as a problem, at all. Especially when one considers that 99% of all FD stations are NOT running legal-limit power, and QRPp (5W) operations are becoming almost the norm (to get the x5 points multiplier).
However, even though RX damage is very unlikely to occur, RX interference from the TX on the other band is quite likely. This depends heavily on how much power is run, and how "clean" the transmitters are. I've found older, analog gear, e.g., an old Ten-Tec Argonaut, Kenwood TS130, etc, is much "cleaner" than modern synthesized gear, in that the old analog equipment uses simple frequency mixing techniques that are unlikely to create much "phase noise" and other evils. My little Ten-Tec Scout 555, although "modern" in nature, and containing a phase-lock loop to control VFO drift, is still NOT synthesized and is extremely clean, compared with the FT1000mp or TS850S/AT that sit next to it.
I'd recommend:
-Plant the two verticals in opposite corners of your 20' square operating space, if possible; this will get them 28' apart.
-Avoid operating close to direct harmonic frequencies; e.g., when one guy's on 7025, the other should not try to operate anywhere near 14050! (This applies even when FD stations are 500 feet apart.)
-Use QRPp (5W), it creates far fewer interference problems and gives you a juicy x5 points multipler and makes almost any station more competitive.
73 de Steve WB2WIK/6
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RE: Spacing between vertical antennas
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by N2MG on June 14, 2001
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Steve wrote:
"I've operated multiop stations for years, running 1500W output power to beam antennas that were stacked on the same tower"
Were those multiband beams or monobanders? In my experience, there's a big difference in this regard. Were there stubs employed?
Of course you are right regarding power levels. 1500W is where most of my attention is spent and it is unlikely to be used as a Field Day power level. Running 100w MIGHT mitigate this problem, but I wouldn't use MY radio that way without being sure.
As far as getting the verticals that far apart (28' in a 20' sq area), they'd have pretty short radials <g> in some directions. But hell, they are already going to have short radials.
73 Mike N2MG
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RE: Spacing between vertical antennas
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by WB2WIK on June 15, 2001
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Hi Mike,
Re the stacked beams: Both, and mixed types.
But this isn't a singular experience, I've operated from M/M stations all over and it's commonplace for Tx and Rx to occur simultaneously using stacked beams on a common tower, and legal-limit power is the norm.
My current multiop station (for operating) is K2XR in NNJ. Stacked monobanders, plus some tribanders, are everywhere. Not uncommon to Tx w/1.5kW on 15m to the 5L 15m mono at 75' and be receiving on 10m, also using 5L mono on the same tower, mounted directly above the 15m beam. An A4S tribander is on a 70' tower planted midway between the 70' 20m monoband tower and the 15/10 stacked monoband tower, with antennas aiming directly at each other and legal limit power used everywhere, with any combination of Tx/Rx bands...happens all the time.
Same site two years ago we had a 6L 10m mono on a short (40') tower, mounted almost directly under the A4S which was on a roof tower just a few feet higher, and using one to Tx on 10m (1.5kW) and the other to "search" the balance of 10m for new multipliers. Zero problems.
We currently use DuneStar electronically switchable bandpass filters in all lines, to help minimize RFI to the receivers, but these are performing just that function: Minimization of RFI. They are not actually "needed," and we've run without them. They just help!
Turns out you only need about 40 dB isolation between a legal-limit Tx and a sensitive receiver to prevent damage. 1.5kW = +62dBm. Any receiver made can accept a 2V signal to its input without damage, this is nothing -- some good receivers aren't even saturated yet at that level. And 2V = +19dBm. Delta between 1.5kW and 2V = 43dB. Not much.
Of course, I'd recommend stubs or DuneStars to minimize interference and actually allow the receivers to hear weak signals in the presence of so much RF. No question. But there won't be any damage without them.
73 Steve WB2WIK/6
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RE: Spacing between vertical antennas
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by N9ZWY on June 16, 2001
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Thank you so much for the replies that have been made. I've learned so much about this topic, and have a lot more confidence than I had at the time I posted this message.
73!
Tod N9ZWY
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