The AM Forum

Most of us know how the height of an antenna (dipole in this case) affects the performance but Im wondering what is the affect on  the receiving performance and can this be plotted.
Thanks,


Bill

The book I read says reciprocity

k

Yes, the antenna will usually have the same pattern on transmit and receive. But you can have an antenna with tons of loss (e.g. Beverage) that would be a terrible transmit antenna and a fantastic receive antenna. When you throw in ionospheric and noise factors, especially on the lower HF bands, reciprocity doesn't strictly apply.

Bill, what are you looking to do? Improve reception of relatively close-in signals (a few hundred miles) or longer distance stuff? Are you trying to optimize in one direction (or maybe reduce noise or QRM from one direction) or do you want better receive in any direction? What band(s)?

(Looks like Huz beat me by 6 minutes...  ;D)


Hi Bill,

As you know, height affects the vertical take off angle of a dipole for both RX and TX.

Then there's the specialized receiving antennas.  TX/RX reciprocity works here too, but receiving s/n is enhanced.

Take a beverage. It might have an overall loss of -20 db compared to a dipole. It's a terrible transmitting antenna. But even though it's down -20db from a dipole, when receiving, the s/n ratio is improved. ie, The desired signals and static noise all drop -20db, but the static noise drops even farther than -20db, resulting in a better listening antenna, also due to directivity and f-b..

So, bottom line is a low height above ground (6'-7' high) can actually improve our abilty to hear desired signals when using the right antenna.

For a dipole, there isn't much improvement in receive s/n ratio when lowering it close to the ground. It just gets deaf. We need to construct a wave antenna (bev) or other loop style antenna (K9AY) to see results.

Hope that covers what you were wondering about?

T

If you don't have room for a beverage, a rotatable loop, maybe around six feet diameter and tuned with a variable cap from a cheap transistorized AM bc rx is supposed to make a pretty good rx antenna.  Even better is supposed to be a pair of 10 or 15 foot tall insulated poles about 50 or 60 feet apart and phased with a variable phasing network.  I have not tried any of this myself but want to (someday).  DX Engineering makes and sells all the stuff for the phased receive whips but I wouldn't be surprised if the information for making all of that yourself is somewhere on the internet. 

Rob

Thanks guys for the info.
Steve as u know because of the lack of real estate I only have a 75meter dipole up about 40 ft.
I know when I plot the ant using eznic the pattern on 75 shows what is expected, a real cloud burner!
I got to thinking (big mistake) about if the antenna receiving pattern was the same. In other words, does the antenna receive signals better if the signals were coming down to the antenna rather than from the sides if u get what I mean. That assumption seemed silly to me so I posted the question.
And JJ......your explanation makes a lot of since .

Thanks again,
Bill     

Bill,

I better understand what you’re axing…  Since you're playing wid EZNec, you probably already know this - if so, then maybe it will help someone else...

An antenna is like a round balloon filled with air. You can squeeze the balloon at the sides and make a standard dipole figure-eight pattern, or form it into a squash and make it a directive Yagi pattern. It’s all done by phasing. (by adding elements, etc)

There’s two views of an antenna – the horizontal and the vertical views. To view the horizontal pattern requires an on-top or bird’s eye view of the pattern. Your dipole at 40’, as well as any dipole at any height, will look like a figure 8 from on-top. That’s why we want the broadside facing our favorite direction and the side nulls away.

The vertical pattern is best viewed from the sidewalk. Picture your 40’ high dipole’s vertical pattern as a light bulb standing on its socket. Much of the energy is beamed (or received) straight up.  This doesn’t mean you don’t receive lower angle energy from farther away stations – it’s just that this energy is down 10-15 db from your higher angle energy. Remember the balloon analogy?  In your case you have squeezed it so it forms a figure 8 from a bird’s eye view and forms a light bulb facing up from the side view. (actually TWO light bulbs slightly tilted, facing opposite directions)

The vertical pattern (light bulb standing on its socket) can be lowered towards the horizon simply by raising the height of the antenna. However, the horizontal figure 8 pattern changes very little with height.  We need to add an extra element or two to change the bi-directional dipole horizontal figure 8 (two directions) - into a unidirectional lobe. (one direction)

Hope this helps.

T

Good visuals, Tom

Rick

The old ARRL antenna book had good drawings of dipoles in the horizontal and vertical planes.  They also had graphs of one at different heights above ground which graphically displayed the lobes Tom is speaking about.  Don't know about the modern Antenna Books, because little has changed and my 1956 book is still valid. 

Tom is lucky he has the room to play with added elements on 75 and 40.  I can hear him down here in the South of Oklahoma quite well when the band is open and he is phased toward me.

Thanks for the report, Jim.

Yep, you're located right in the western path of the pair of 75M delta loops at 190', Jim. It's now switchable to the west.

I'd like to get on this weekend and work some coast to coast AM. I'll do a post here when I do and maybe we can talk.

The last time I tried it a month ago it started out well with some west of the Miss River stations and even Bill in Colo called in. Calif never showed cuz we didn't stay on late enuff.

Maybe the corndix will be good enuff to do it soon.  We like to make the transmissions short and get as many stations involved as possible.  As long as we keep axing for new DX AM check-ins and everyone keeps the  comments brief, it works out FB.

T

Daughter's birthday and anniversary so will not be around the radio this weekend.  However, with me hearing WWV @ 2.5 until nearly 10 AM and starting at about 3 PM says we are going to have at least moderate to long distance this winter.

If last night on 160 is any indication, you will have a gooooood time.  75 was so long that I couldn't hear stations within a 500 mile radius of my home after 8 PM last night and the night before.  A storm front with possible severe storms is due tomorrow night and it will work its way East Friday and Saturday.  We shall see.

I have a 10 meter vertical on a tower.  Its about 85 ft up.  Many times, It is the clear winner for RX over my Double Zep horizontal at 55 ft.  Last night I had two rigs on listening to 3885. One on the 10 meter vert and one on the Zep.  The peak signal was very close.. Within 5 DB.  The noise on the Zep was S9.  The noise on the Vert was S1.  Big difference.  It allowed me to actualy hear a couple of you guys on this forum that like to run a HUGE carrier with almost no audio :)


Clark

Those death antennas are killer.

You can compensate if they excite a low angle death ray

My SUPER Death Ray 10M antenna Did it again!    I heard a Certain Board member with his HUGE carrier and Low audio on last night again..  Was able to pull him out on the 10 meter Death ray.  Then another board member came on and about blew my speaker out.. LOL.   

Whats neat is I can use BOTH antennas at once and fade between them. This seems to really help :)


The 10MDR is really neat on Noisey nights!

Its interesting listening to East coast stations now that the band is longer.  You guys really hold the key down.. LOL.  I was doing some work and thought it would be fun to Time some of you guys.  The winner was 31 minutes!!!!   Now thats an old buzzard! Most seem 10 to 15 minutes lock down keyup is normal.  Different style as to what takes place over Here.  More of a Say what you want to say and unkey type of Round table.

Clark

Those death antennas have a fatal flaw, though. :D


Sounds like you're simply trying to determine if the reception you get now is to be expected, Bill?  If you're looking to improve your reception over what the dipole yields but are space restricted as you seem to be saying, check out the receiving set up that Steve uses at his place. It consists of a pole maybe 10 feet high with wire loops attached. Takes up minimal space, but sure helps pull out the weak ones. Rather than boosting the signal greatly, it instead cuts down the noise considerably. Signals in the static suddenly become good copy.

I will have to look into that. I wonder if I could use my Inverted L for that purpose?  Any plans for making such a tuner?

C

I saw an MFJ box that used a seperate antenna to null noise out.  I will do some research.  Sounds like a neat project!

C

Clark:

The MFJ box is good for nulling out local noise/interference (crap from your neighbor's computer or plasma TV), but is generally not good for removing noise propagated in from a distance, like lightning static, etc. THis is because the angle of arrival for the distant stuff is not constant, so you could have it nulled out for a second and then the null is gone. Locals stuff is groundwave and very constant, so the nulling works.

Some 160 meters guys have used the MFJ box or similar to enhance a particular signal, but even then the results are transient. So, setting and forgetting is not likely to happen.

Invest your time/effort in a good directional receiving antenna or antennas. You'll be shocked how much better you can hear.

Thanks for the advice.. How about this system nulling the Guys Pool pump in the summer?  That would be great! 

I dont have the room for Directional antennas on the low bands.  I am pretty much maxed out with my Zep.  One can dream though.

Nulling out T-storms? Correct... it's like trying to hit a moving target. I guess it's cuz T-stoms often are in many different areas at once.

Jay/W1VD sent me some torroids and other small parts about a year ago. I built up a phasing unit that would take two 50 ohm antennas and phase them from 0-180 degrees, respectively. I used the NE quads and the west Yagi on 75M.  I could null out radio signals to beat the band. Could drop them into the noise. But the T-storms were a nightmare. I'd get a moderate null, then the next crash wud come in max strength. I'd null that one and then it wud change again. Plus, try to null a static burst!!  It takes an impossibly fast reaction... ;D

The DSP stuff out there today is also about useless for static crash nulling, so I figggered if this method worked, I'd have quite the receiving edge. But alas, there appears to be no method for nulling out T-storm crashes. Maybe someday a sophisticated neural network will do it. But for now we must use our own between the ears, caw mawn.

T

Collins made a little unit that plugged into the 75S series of radios for noise cancelling.  It monitored ~ 50 Mc because they found that local spikes and such were stonger there.  The best thing I have found for static is a rotatable loop or beverage.  Usually the station I want to work is located near 90 degrees from the noise, and either the loop or beverage will help reduce that noise.

Yep, agreed, Jim.

I found the best lightning suppressor is a SINGLE antenna with a big front to back.  

Why a pair of phased antennas (as I described in my previous post) will not do the same trick, I dunno.  I mean, a Yagi is a set of phased elements that rejects lightning noise off the back and the rejection stays stable - why shouldn't a pair of phased separate antennas do the same with a similar pattern?  Doesn't make sense to me.

For example, tonight I hear 75M is very noisy to the west with what sounds like lightning crashes. When I switch to the NE, the noise drops a good 12-15db.  I can hear both ambient AND crashes that are similar in attenuation.   Using a pair of phased antennas with the phasing box will null signals, but not the crashes.

I must be thinking something incorrectly.

All I can figure is that the Yagi's vertical pattern off the rear is fairly broad - while the phased antennas are a sharp null, thus unstable.

T

I have tunability.  One antenna is into the main RXer.  It has bandwidth and filter shift.   The vert is on the other sub reciever with inrad filter.  I can then MIX in any mount of each antenna.  Kinda neat. 

I noticed that some stations really come in strong vert.. While others come in strong on the horizontal antenna. Probably because of the antenna type the other station is running?

I am not much of a DXer so all of this is really kinda pointless.  It would be nice to Null out local noise in the neighborhood.  Thats all I was thinking about really. 

I also realized that the vert was running the reciever at wide open.  While the Horizontal had its preamp off.  This accounts to why the signals where close to the same peak on both antennas.  The vert was on a strong reciever and the horizontal was on a weak one.  Still.. The noise is MUCh much less on the 10 meter Vert.

C

Check out the K9AY. It does not take much space. I bet you could fit one in your yard. They are easy to build and I've used one since 2006 (wish I had built one sooner). My Beverage is better but for the space required, the K9AY is hard to beat.

And yes, you can null out local noises.




That's exactly it. It's not geometric F/B but the entire back side of the pattern. Those are the best antennas to reduce noise/static.

can't null out t-storms because lightning strikes are always at random distances and directions so the phase relationship is always changing. 

I have found the only thing that works with them is high power carrier, and good antenna, not at your QTH but at the station you're working.  Then all you have to do is turn down the RF gain.  :)

This works great with 50 KW broadcast stations laying down a blanketing groundwave from a 190 degree tower, which squelches storm static into punyness within a certain field strength contour, but hams are much more power challenged  :-[

Rob

What Rob said and also what HUZ said about the K9AY.  I played with one and you really will like it.  The only problem with buying the box and controller is the price.  It does do a very good job if installed over radials.

There are other small options too. As Jim mentioned, a small "shielded" loop can put nulls and T-storms at certain distances.

Also, don't think that the stations need to be DX or similar for the K9AY or a Beverage to be useful. I see nice SNR improvements on stations less than 300 miles away on both the K9AY and the Beverages. I'll post some audio clips later.

Here are some audio clips. The first is of Kerri - KC2UFU on 3872 kHz the other night. She is located 211 miles from me. The static is not terrible but you can hear the noise drop when I switch from the dipole to the Beverage. The clip starts out on the dipole, then switches to the Beverage at about 12 seconds, then back to the dipole at 27 seconds and then back to the Beverage at 32 seconds.

The second clip has Kerri and Bob - K1KBW. Bob is located 352 miles from me. Here you can hear the reduction of some SSB QRM. Both Kerri and Bob are located Northeast of me, right in the main lobe of my Beverage. The SSB stations were all to the South of me. Some were in Florida. Notice the drop off in the SSB when switching from the dipole to the Beverage at 8 seconds. Then the SSB comes back up at 22 seconds when I switch back to the dipole and drops again at 30 seconds when I switch to the Beverage. Then it's back to the dipole at 44 seconds and back to the Beverage at 52 seconds. The drop off in the SSB is completely due to the back and side rejection of the Beverage. I'm not changing IF bandwidths at all.

I have a tuned loop on the back deck for 80M. It is a 4 ft diameter loop of coax  with a split shield and resonated with a variable at the feed with a split C match. I am having issues matching it efficiently to 50 Ohms. The beverage is kicking it around the block for signal strength.

I am sure that this antenna will work better once I figure out how to match it.

With broadcast loops I have used a few unshielded turns of hookup wire around  a similar form tuned with a cap and a Faraday loop of about 1 ft in diameter directly feeding the coax roughly in the center and this works great for sensitivity and it has a sharp null.

Mike WU2D

Hey Steve I don't sound half bad with my 35 watts carrier power.  I guess if everyone had your antenna setup I wouldn't have to burn my fingers building this amp!

Now I know what I sound like, thanks!

I used to SWL in a basement apartment many years ago. I'd get great reception from a piece of wire strung across the ceiling!

This info here is telling me that I'll have to sharpen my skills for receiving, if we move to the Philippines.
I think I understand why they (especially  Europeans ) use mag loop antennas. Antenna space is limited and the tropical location will get pretty noisy from MaNature. Noise cancelling will work somewhat, but like Tom says, a crap shoot.


Fred