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Old 18-02-2018, 06:23   #1
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Is length of antenna for MF/HF frequencies (Rx only) important?

Hi Guys,

I think I understand that power output and length, angle and placement of antenna are important features for the Tx side of an HF/SSB set up. But just how important are they for the Rx side of things?

Let me explain - whilst being able to talk from anywhere in the world via a radio makes good practical sense for some, it isn't a hugely important factor for me. There are also the cost implications, power draw and, well, just more technical kit to go wrong. I'll have a mobile sat phone if the proverbial hits the fan! That said, I'd like to be able to receive HF/SSB signals so that I can grab weather faxes.

It may be pointed out to me that I can grab grib files via the sat phone. I can. But that is costly data and I'd like to have redundancy in my comms system. Belt and braces

Several options spring to mind after some reading and researching:

1. Basic HF/SSB radio installation.
2. SW radio capable of tuning down to the lower range of frequencies plus necessary gubbings.
3. RTL SDR dongle with a small whip antenna.

I guess my questions are these:

- do I need a super big whip, dipole or backstay antenna for receiving only on a conventional type radio set up?

- Just how good/reliable/capable are the SW radio/dongle options if you take the middle of the Pacific to be the worst case scenario - the remotest place I am likely to be.

Hope that all makes sense

Cheers in advance,
SN.
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Old 18-02-2018, 06:46   #2
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

Hi Nomad,

I'll give it a shot - it depends...

The longer the antenna, the more signal it will gather - but on HF, natural noise is a bigger part of the equation than signal strength itself, particularly at the lower end of the spectrum. There are perfectly usable HF receiving antennas out there that look like a masthead VHF antenna with a somewhat longer whip, though the best ones have a small amplifier added to make them more effective. You've probably seen the older Loran and Differential antennas that work below 500 kHz - these work on the same principle, though they don't work well for HF, unfortunately.

From the perspective of emergency availability, a portable HF receiver makes a lot of sense, and these typically come with a telescoping whip, so you can sit on the beach or up in the cockpit and tune in what you're looking for. I would make sure you can power it from the boat as well, of course, unless you like hauling lots of batteries.

I am not personally familiar with the SDR Dongles, though I know folks who feel they are a great thing - I do know they suffer from problems when confronted with strong signals nearby to the signal you're after, and this is a common situation on HF - better you should get advice from someone familiar with them for solutions in that regard.

Your biggest enemy in all three scenarios is locally-generated noise, either from your own boat (computer, generator, solar regulator, etc.) or things near you (marinas are the RF noisiest places on earth!) - if you are hearing more noise than signal when you're out and away from other electrical stuff, try turning things off, disconnecting solar panels, etc. to see where the issue is. Note that an SDR dongle HAS to have the computer turned on!

Good luck!

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Old 18-02-2018, 06:57   #3
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

I have the SDR Play, its alright.
Depending on the freq you want to receive the db gain drops as the freq increases.

there again depending on what u want to receive, VHF weather satellites you can pickup and decode, but u do need a computer, a small hand held beam antenna and a couple of pieces of software,
HDSDR and wxtoimg.
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Old 18-02-2018, 15:47   #4
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

The behavior of an antenna on receive is the same as on transmit. The more effective an antenna is on transmit, the more effective and efficient it will be on receive.

The difference in outcome with an antenna on receive compared to transmit is really the difference in the amount of GAIN available in the receiver or transmitter.

Then transmitting, the transmitter power is usually limited and fixed. If you apply a certain power to the antenna, say 100-Watts, a poor antenna, no gain, poor efficiency, will radiate a weak signal. You cannot add any gain to your transmit. You are stuck at your limited power.

When receiving, the receiver itself has a very large amount of reserve gain. The gain is not fixed. The receiver can increase gain as necessary. Typically a receiver might have a reserve gain of more than 1,000,000 (60 dB). This means that although the signals coming from a poor antenna are much reduced from what they could be with an excellent antenna, your receiver can make up for the reduced level with more gain.

Only when the receiver runs out of gain and is operating at full gain will the receiver be at a limit. At that point, reduced signal level from the antenna will begin to show up.

A further consideration is the expected level of signals you want to receive. If you just want to be able to copy some mega-Watt broadcast blow-torch signal, you can probably receive that by using your body as an antenna. If you want to copy a very weak signal which is near the ultimate sensitivity of the receiver, you will need a good, efficient antenna with at least unity gain.
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Old 18-02-2018, 17:51   #5
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

Quote:
Originally Posted by settlednomad View Post
Hi Guys,

I think I understand that power output and length, angle and placement of antenna are important features for the Tx side of an HF/SSB set up. But just how important are they for the Rx side of things?
Nowhere near as important on receive - get a long bit of wire up there and start playing. For weatherfax I've actually had surprisingly good images just using a degen1103 ssb receiver with the built in antenna resting against one of the chain plates down below then n android tablet sitting next to it running a wfax app. That was across Biscay. SOmething like a Degen1103 might be worth considering rather than an SDR, the cheap RTL dongles are great but don't receive low enough to get weatherfax - which is the No.1 thing you want to receive offshore IMHO.
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Old 18-02-2018, 18:38   #6
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

Quote:
Originally Posted by settlednomad View Post
Hi Guys,

I think I understand that power output and length, angle and placement of antenna are important features for the Tx side of an HF/SSB set up. But just how important are they for the Rx side of things?
They remain important, however, some aspects of antenna and feedline design are much more important for tx than they are for rx.

There is a shortwave listening (SWL) "community" that is active and passionate about what they do. You may be able to find resources specific to them.


Quote:
- do I need a super big whip, dipole or backstay antenna for receiving only on a conventional type radio set up?
That depends on what you're trying to receive and how reliable you want your reception to be. A long whip or backstay antenna will certainly work, and since impedance matching isn't important for rx, you will not need an antenna tuner.

But there are other designs that work well for rx that are ineffective for tx because of impedance and efficiency. Typical consumer MW receivers have terrible antennas and they still work. They work far better with a 3' diameter coil of wire. Bigger is better. You will also pick up all the RF noise in your cabin unless you elevate it, which is one of the advantages of using the backstay or a whip.

I would suggest getting a good quality receiver (like this) and experimenting.

I haven't used the new SDRs. I will observe that the only place with more RFI than a marina is a server room. Computers emit all kinds of RF, so you'll want to maintain distance and isolation between your antenna system and the rig if that's the way you go.
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Old 18-02-2018, 20:34   #7
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

A simple way to test if your antenna is adequate for receiving is to listen to the background noise with the antenna connected and disconnected. If the broadband "hiss" gets noticeably louder when you connect the antenna, then it's generally going to work well enough.
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Old 18-02-2018, 20:59   #8
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

Unless in a metal yacht, simply connecting the external antenna terminal on the receiver to a handy chainplate will usually provide a useful antenna. It is possible that this will result in excessive RF noise pickup, but I've used it successfully in many boats.

An inexpensive experiment to try!

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Old 19-02-2018, 02:22   #9
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

I would not subscribe to the view that "longer" is better.

Most random wire antennas like backstays or sloping dipoles have a very good efficiency. There is little point in having the best efficiency if your radiation take off angle is too high. In general for SSB communications, the takeoff angles between 5 and 15 degrees are of greater importance than the high angle radiation which is generally 30 degrees and above. These high angles are useful for NVIS communication thats typically used for coastal communication range from 4 to maybe 12mhz.

Ideally one would have an antenna that accommodates both low and high angle radiation, which is impractical for most yachts. The length of the backstay will always be a trade-off. Its well accepted that 5/8 wavelength at the highest frequency of use is about the optimum length for general purpose use. If you a ham equipped yacht 14mhz is generally the highest frequency that most yachts would use. In tropical areas this may be higher.

So a 5/8th wavelength backstay length, is generally around 40ft long. When this 40ft backstay is analyzed from 4mhz to say 14 mhz its performance is just about optimum when used with a general purpose antenna tuner. Its impedance is neither low enough to cause considerable loss and stable enough across the range upto 14mhz. There will be a typical frequency where this 40ft length will be a half wave high impedance feed point which can cause problems for some tuners. With the typical interaction with yacht rigging you will struggle to guestimate this length with just simple formulas. Its never right. After having installed hundreds of backstays, I know from using a very accurate Network Analyzer that the half wave impedance point calculated by formula is wrong 100% of the time.

A backstay that is too long that is typical with yachts over 45 feet you just using the backstay as a random wire antenna whose radiation pattern is far from optimum for long distance SSB communication and things like Pactor. Yes everything works but if you want the best performance its best not to go too long. An over length backstay can reduce your signal as much as 20db based on my anecdotal comparisons and shootouts using my yacht as a reference. A super long backstay would have been great a few decades ago when most SSB communications was on the lower frequencies are rarely did yachts operate above 8mhz. 5/8's wavelength on 8mhz is 73ft for example.

If you heading into the Pacific, you want a general purpose antenna that offers good low angle radiation performance on frequencies from 6 to 14mhz. Coastal stations are rare as hens teeth in the pacific especially active ones. The ability to propagate your signal to distance stations is therefore of critical performance.

So thats the long and short of general purpose backstay antennas which operate generally as vertical antennas that have very good efficiency. I would encourage you to pursue a backstay antenna using something like Stalock insulators. Antennas should be optimised not "just" to have any antenna or any piece of junk wire but it should be optimised for the takeoff angles that support the type of communications you want to do.

If you give me your mast height, backstay length, and Yacht rig configuration I can quickly run a simple model and see what performance is possible based on length.

Its amazing what you can do with a simple backstay and by simply grounding various parts of the rigging, you can have a dual purpose high and low angle radiation, just by breaking a connection with a simple toggle switch. Let me be clear about this, a yachts mast always has a massive impact on the radiation pattern of the typical backstay. The side effect is always detrimental and these effects is beyond most sailors understanding of how antennas work. If you do understand these impacts and how to control them you can use these so called interactions for good antenna performance. I wont complicate the subject matter too much because we might just end up with another endless antenna debate about "my length is better than yours" Always stress takeoff angle which is generally very good for a yacht over sea water. This aspect of antenna performance is rarely discussed in yacht SSB forums which is no surprise.

If you want a quick easy fix do this. Run 1 piece of wire as a radial from the stern to bow. Connect this radial wire to your antenna tuners ground lug. Connect any of the rigging to the antenna terminal of your tuner and tune across all the bands. if your yacht is fiberglass this will work perfectly well with no effort. The ignorant and the perfectionist naysayers will spin endless crap about why this wont work, but you wont have any difficulty communicating with most coastal and other yachts. I have installed this configuration on 20 yachts for "poor sailors" they all happy. Sure on some frequencies and extremely long distance communications the performance "might" be less that optimum but it will work. Get a good tuner like the SGC230 and try this you will be pleasantly surprised. Theres many ways to skin the proverbial cat in life.

Good luck,,,
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Old 19-02-2018, 03:11   #10
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

You need an antenna tuner so your Tranciever sees 50 ohms at the radio ,,
then you wont cook yopur TX transistors , at the same time the RX will be at optimum ,

I use a MFJ tuner , there cheap easy to use and you can tune it (rough) in RX
well worth $80
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Old 19-02-2018, 07:57   #11
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Is length of antenna for MF/HF frequencies (Rx only) important?

Some of you guys are missing the point that the OP is talking about a RECEIVE ONLY antenna. As such, a simple random wire (which might include a shroud or even lifeline) is going to be just fine. As far as possible from sources of RFI, like inverters and Danfoss compressors, is best.

Forget about the complications of resonance, antenna tuners, and take-off angles for a receive only application at HF.

Chip
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Old 19-02-2018, 08:37   #12
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

SDR dongles work great. Like all receivers the better the antenna the better the result. I use mine with a pizza pan as a ground plane with the magnetic mount antenna that came with it.

About $25 on Amazon. Plugs into the USP port on laptop.

Use SDR# software, free!!
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Old 19-02-2018, 11:05   #13
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

All,

Thanks for taking the time to respond. As mentioned, it is only for Rx. As I am a belt and braces man - and I like to experiment with things - I think an inexpensive dongle has to be worth a bash. A few of you seem to have achieved great things with very little. Those are my kind of results!!

I'll also keep an eye out for a relatively inexpensive secondhand HF radio on Ebay or such site. I'll get some wire and start playing around with it. I still have some mates in the Signals platoon who may be able to half-inch an old dipole (just for the strands of comms wire).

Point taken on the placement of kit within the vessel to avoid interference.
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Old 19-02-2018, 11:53   #14
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

buy a $2 transistor radio ,,,
If not as i said get an antenna tuner ,, YES for RX ,

There is no other way unless you want to get a piece of wite at the exact resonace of the frequencey you want to listen to ,, then trim(YES TRIM) it to the new frequency ,,

Or you could get an antenna tuner (yes again) to TRIM the antenna for you ,,,

ITS called resonance ,,
Google it ,,,
Its basic electronics ,
RX TX it dont matter ,,,,,,,,
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Old 19-02-2018, 12:01   #15
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Re: Is length of antenna for MF/HF frequencies (Rx only) important?

That point is not missed. Its the mistake made very often by many assuming that the RX only and TX antenna efficiency is 2 different subjects.

The junky pieces of wire that some yachts people use on the assumption that its good enough is appalling. Installing any receive antenna for low noise and good performance can be more demanding than installing a transmit antenna, thats if you want a proper receive antenna. It also does not help when today boats have tons of junk electronics that can cause common mode noise.

A highly efficient receive only antenna that has an optimised pattern for RX will work as a great TX antenna assuming its efficient. If you go through all the trouble you might as well just connect a transmitter.


Quote:
Originally Posted by SoonerSailor View Post
Some of you guys are missing the point that the OP is talking about a RECEIVE ONLY antenna. As such, a simple random wire (which might include a shroud or even lifeline) is going to be just fine. As far as possible from sources of RFI, like inverters and Danfoss compressors, is best.

Forget about the complications of resonance, antenna tuners, and take-off angles for a receive only application at HF.

Chip
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