The following discussion about open wire feeding and baluns took place in the Moon-Net reflector. I found the subject so interesting that I decided to put together all the messages in this page.
On 4-Oct-2004 ON4DPX wrote:
A few weeks ago I saw the post on moon-net about EA8FF (http://n1bug.net/tech/ea8ff/713xy.html)
his 70cm array . I was very interested in the system especially the 4 yagi array
. Now I'm considering for open wire feeding , there are articles that don't
recommend it if you live in area with various weather like ON and that the
difference between open wire and coax isnt that big then there are article that
tell it otherwise ...
Now I would like to know if its worth using open wire feeding in a various weather qth and if its worth open wire versus coax ? Anyways I will build the 4 yagi group from EA8FF but I still dont know what feed system I will use .
Who can help me ??
On 4-Oct-2004 I5CTE wrote:
Hi Kenny, I've had quite a bit of experience using open-wire
lines on a 4 yagi group for 70 cms over the last ten years.I have been very
happy with them;admittedly,weather at my location is rather more clement than at
yours,but it can be very wet here too if it sets its mind to it.The main
thing,in my experience,is to keep the return loss,under dry conditions,as low as
possible,and I mean 25 dB and up.That will keep the SWR at acceptable levels
even in rainy weather. Best of luck
On 4-Oct-2004 ON4DPX wrote:
Thanks all for nice respons .
The choice has been made so I go for open wire feeding . But I still have some other small questions .
1. Does the lines need to run straight , the photos of EA8FF show that the lines arent running straight what would be the difference ?
2. Does the spacers need to be Teflon or can I use other plastic non HF conducting materials ?
3. If I solder the lines to other lines with T joint would there be a difference in SWR if the T joints have a larger diameter the the tubes from the feeders themself ?
I forgotten to ask something about the spacers . Is there a certain rule of thumb about the quantity of spacers or can you just place that much like you want for mechanical stabillity ?
On 4-Oct-2004 WA6PY wrote:
Hi Kenny, Long time ago, when I was in Stockholm I was using
4 x DL9KR yagis with open wire feed with great success. The weather was very
often bad. I had slug tuner to compensate for VSWR, but even without it detuning
was not bad. In my case feed lines were not very strait. Important is distance
between lines. I was using PTFE as spacers. Jan DL9KR pointed in his original
description, that it is very important to place spacers in the current nodes,
and then moisture will not affect VSWR that much. Originally I was fine tuning
open wire feed for minimum Return Loss by pushing together or spreading line
symmetrical close to the center feed point.
On 5-Oct-2004 W7QX wrote:
I ran open wire feeders to my 4X20 foot yagis for several
years with very good results.
I designed my yagis for a 200 ohm feedpoint impedance using the step up impedance folded dipole approach which was 25 ohms times 8 = 200 ohms. The feeders were equal in length but to no wavelength factor. The feeders formed an "X" shape and were fed with 1/2" Heliax thru a Bazooka balun where they all joined together.
As to the open wire itself.... I used #10 enameled wire with teflon spacers (1/4" teflon rod) spaced about 3.5 inches apart. I would use nothing less than teflon insulators!
I made a Jig which permitted me to cut and drill the spacers at about 15 per minute. The most effort was threading the spacers onto the wire.....it was a "press" fit so the spacers would not slide.
Rain did not bother the open wire feeders, but if there was no wind and it stopped raining and raindrops would hang off the ends of the directors the VSWR would increase until I shook the antenna a bit to get rid of the raindrops.
The lower yagis were about seven feet above the ground and with 1500 watts into the antenna, I could draw up to a 3 inch arc off of the first directors using a wooden pencil connected to a one foot piece of polystyrene.
I recommend the use of open wire feeders and N4PZ is currently just getting his 8 yagis going using my feeder approach.
On 5-Oct-2004 DL4MEA wrote:
Hello Kenny,
have a look at my design: http://www.qsl.net/dl4mea/antennas/pwirefed.htm
The wires don't need to run straight, they even do not need to be same length. This array work(ed) ufb, even that it still exists, there is no longer AZ/EL control. I?ve once worked N1BUG when he had a single yagi.
For bad weather conx I suggest to make the parallel wire so that it does not transform any impedances.
On 5-Oct-2004 SM5BSZ wrote:
Hi Kenny and all,
I think open feed line should be very advantageous, but I also think it is possible to fail when doing such a design. SM5BFK had a big array of small yagis on 432 MHz many years ago. It was not very sucessful. When he replaced it with a 5 m parabola which should have a similar size of the effective area, the results improved greatly. I do not have details, but something like 6 dB if I remember correctly.
I do not know what the problem was, it was never discovered, but it can not have been the antennas themselves, it must have been the feed system that used a large number of open feed line sections.
I think you will be sucessful and get better results with open feedlines compared to what is possible with coaxial cables if you are careful with the spacers.
There are two kinds of problems with open wire that I think you have to worry about:
1) Radiative losses. 2) Dielectric losses
I do not think unequal feed voltages, SWR or ohmic losses should be any problem.
Have a look at: http://www.qsl.net/dl4mea/antennas/pwirefed.htm The quarter wave sleeve balun here is very important. The voltages applied to the two tubes forming the 100 ohm feed line must be accurately equal in amplitude and 180 degrees out of phase. This may (will probably) require tuning of the sleeve. Just cutting 0.25 wl according to the data book will probably not provide a perfect balance. The reason is the large stray capacitance between the end of the sleeve and the nearest tube of the open line. The capacitance may well be more than 1 pF and that would correspond to a coupling impedance below 350 ohms to ground for one of the tubes and no such coupling for the other tube. It has to be compensated by a shorter sleeve.
We may assume that the impedance for feeding 432 MHz into the midpoint of one of the tubes while feeding nothing into the other tube is several hundred ohms and that the impedance for feeding current on the outside of the sleeve and further down the outside of the coax is also several hundred ohms. With luck you might get an impedance of 1000 ohms in total with all three impedances in series. You may be lucky and get a little more, but you may as well get much lower. The impedances are complex numbers and the reactive parts may cancel.
Assume the impedance is 1000 ohms. That means that your receiver looks into 50 ohms (the antenna) with T=30K which is in parallel with 1000 ohms at 270K. The signal from the moon will deliver 1/20 of its energy in the 1000 ohm load which is a 0.2 dB loss that you might tolerate. The coupling to a parallel mode excitation on the open line and a current on the outside of the coax could produce a low temperature if you are lucky to have the corresponding radiation hitting the sky. Worst case is that all of it is losses and lobes pointing to ground giving a 270K temperature. If your system noise temperature is in the order of 60K, there would be an additional loss of something like 0.8 dB.
Not tuning the balun would probably cause a loss of sensitivity of something like 1 dB. You may be lucky and get much less, but I think the risk is pretty high that you get several dB loss of receive performance.
Tuning the balun is very easy. Take a thin coax (2mm or so), run it tightly on the outsideof the sleeve and connect the screen to the edge of the sleeve. Run a thin wire or a resistor from the center conductor of the thin wire to the center conductor of the main feed coax. Inject a signal into the thin coax with a signal generator and find out at what frequency there is a sharp minimum. This is the resonance of the sleeve balun. Most likely it is too low. You can shift the resonance upwards by squeezing the shorted end of the balun. A smaller distance between screen and center conductor causes a lower inductance in the resonator which leads to a higher resonance frequency. If you want to shift the frequency downwards, squeeze the open end of the sleeve. Here a shorter distance causes more capacitance and lowers the frequency. (A good idea is to simulate on the lab bench....)
There is a description at: http://antennspecialisten.se/~sm5bsz/antennas/cpudes02.htm Figure 1 and related text.
Radiative losses may be introduced anywhere in an open feed system if you allow non-symmetrical mounting that makes the two wires see different capacitances to the surrounding. Except for the balun I do not think this kind of asymmetry is of any concern.
The other problem is dielectric losses. Be very sure to avoid lossy dielectric materials. I suggest you use teflon because of the weather resistance. I am sure there are other materials that are as good electrically or even better, but be sure to avoid PVC or plexi-glass (which probably work fine on HF bands)
The spacers could be the dominating source of losses. Do not use more spacers than necessary. If the precision in wire spacing is poor, the impedance will be somewhat incorrect but there will be no losses for that reason. Impedance inaccuacies may cause some difference in how much power each antenna receives, but modest unbalances are harmless for system performance. Modest impedance errors will of course cause some SWR, but on a loss-less system, SWR is harmless and can be tuned away at the main feedpoint.
Do not use more material for the spacers than really required. Every cubic millimetre of plastic that is subjected to a strong RF field (a point of high impedance) will introduce losses.
If your wire (for 300 ohm open feed) is the same size as RG8 (2.2mm) you can estimate the losses like this: RG8, 6dB/100ft @450MHz RG8, foam, 2.8dB/100ft @450MHz (from 1994 ARRL handbook)
It means that surrounding a 2.2 mm wire with plastic causes dielectric losses of about 4dB/100ft at an impedance level of 50 ohms. In the open feeder with 300 ohm impedance, each wire is 150 ohms with respect to ground. This means that the electric field is sqrt(3) = 1.73 times higher while the current in the wire is 1.73 times lower. The dielectric losses caused by surrounding one of the wires in an open feeder with plastic would be 12 dB/100ft because losses go with the square of the electric field. A 300 ohm balanced line with plastic dielectric everywhere would give dielectric losses of 24 dB/100ft or 0.79 dB/m.
What counts is the total length of your spacers. Make it as small as you can, imagine pushing all the spacers together and look at the short, full plastic, transmission line you would see. Would it be less than 2 dm, fine. A large part of the electric field will be outside the plastic so the losses will most probably be 0.1 dB or so (but remember it is room temperature so it is still a factor to take into account for the receiver)
There is one thing I do not know about open feeders and that is the significance of corrosion protection. In case you use enameled wire, there are different kinds of laquer they use. It will protect from ohmic losses growing with time, but it might affect the dielectric losses. We have started to look into it at Antennspecialisten because the same problem is relevant to antenna elements. There are no results yet, but they will become available on the Internet when the study is finished.
I am sure there are moon-net readers who know the formulae to compute ohmic losses per meter and dielectric losses per meter for a balanced transmission line in a dielectric medium. The comparison with RG8 was just to create some kind of feeling for the mechanism and the size of the effect.
In the same way we may guess that the losses of an air dielectric RG8 is in the order of 2dB/100ft. Most of it is on the center conductor, the screen has a much larger area. The losses on a single wire of the 300 ohm line would then be 0.66dB/100ft so the balanced line would have a loss of 1.3dB/100ft. This is about 5% compared to the dielectric losses for an open line in plastic according to the above discussion. It means that with spacers covering 5% of the open line, the losses are twice as high as for the ideal line. With 1% of the electric field inside plastic which I think is fully realistic, the losses should be in the order of 1.6dB/100ft or 5dB/100m.
The above discussion suggests that a well designed open feed system with 2 mm wires at 300 ohms will have similar losses as a 1/2" foam dielectric (Cellflex) cable. With 4 mm wire the open feeder would correspond to a 1" Cellflex. Which one would you prefer for the stacking;-)
There is one more thing. Avoid placing the spacers at regular intervals unless you make sure that the interval does not match any multiple of 0.5wl. Each spacer works as a small capacitor and the effect of a single one on the feed point impedance is negligible. If they happen to sit on electrically equivalent positions, they all come in parallel at the feed point which may cause SWR. Maybe it is a good idea to use spacers pair-wise, separated by 0.25wl. Such pairs would fix the separation as well as the direction of the rods forming the feed line and they could be made from thin plastic. The SWR effect would be zero because the two members of a pair cancel. The distance between such pairs could be made pretty long.
There is another thing about open feed systems. Water. I do not know how bad it is, but it should not influence SWR much on a well designed system - but I am afraid it would increase losses by a large factor. My feeling is that losses would increase by a factor of 2 to 5, but if the losses are really small to start with, it is not a serious problem. (Would be interesting to read something about galactic noise as seen by dry and wet arrays)
Wet snow would make an open wire system useless. Does not matter much because yagi antennas are useless anyway when covered with wet snow:-)
If you had the patience of reading all the above, please correct me if you found mistakes. Open feed vs coaxial cable is an old discussion and the SM5BFK failure is still not explained. There is some need for both practical engineering and theoretical understanding. It seems to me that 432 MHz yagi arrays for EME will use open feeders in the future and that currently used designs are basically fine.
On 6-Oct-2004 Dl4MEA wrote:
Hello Leif,
thanks for the absolutly fine analysis. I will consider this whenever I do another array. But this 6 yagi array worked fine and did not require rougth tuning. But you may be right with your comments.
Anyhow, I would never make it like this again with a 100ohms line between, instead I would connect the 6 lines directly to the short coax piece. Nevertheless there would be a kind of stray capacitance, so your procedure would apply there, too.
Additionally it should be analyzed if the balun could be made from ferrites, like I did on 2m, suggested by Graham, F6VHX, during his work for the straigt dipole feed for the DJ9BV yagis. That would avoid such effects that you mention for the bazooka balun, I assume, but I am not sure about other side effects. Actually on my 2m yagi that works fine even with QRO.
On 6-Oct-2004 N1BUG wrote:
Hi Leif and all,
This is becoming a very interesting topic. I was especially interested in this: (I have a question below)
> Have a look at:
> http://www.qsl.net/dl4mea/antennas/pwirefed.htm The quarter wave sleeve
balun here is very important. The
> voltages applied to the two tubes forming the 100 ohm feed line must be
accurately equal in amplitude and 180
> degrees out of phase. This may (will probably) require tuning of the
sleeve. Just cutting 0.25 wl according to
> the data book will probably not provide a perfect balance. The reason
is the large stray capacitance
> between the end of the sleeve and the nearest tube of the open line.
The capacitance may well be more than 1 pF and
> that would correspond to a coupling impedance below 350 ohms to
ground for one of the tubes and no such coupling
> for the other tube. It has to be compensated by a shorter sleeve.
OK, that makes sense. I am now working on a 432 system with open wire feed. I will have 200 ohm impedance at the array center part where I need to convert to coax cable. I plan to use a 0.5 wl coax "U" balun at this point. It seems like this might not present as many problems with stray capacitance as the sleeve balun, but there may be other problems I have not thought about. Do you have any comments on this arrangement? Is this a bad idea?
On 7-Oct-2004 GW4DGU wrote:
I spent a happy few hours looking at real-life baluns using
my network analyser when I was developing what has turned-out to be a very
successful 70cm yagi a few months ago. The balun work isn't complete, as I got
the results I wanted, and I was doing the work for myself without thoughts of
publication. But, I came to a few conclusions based on my measurements which
seem relevant to the current discussion.
Firstly, it was quite difficult to get good symmetry from a bazooka balun. It's not a symmetrical structure, and the fringing capacitances of the inner and outer are significantly different, as has been noted. In particular, based on my measurements, I wouldn't try to use one as a 200 to 50ohm transformer.
Despite its popularity, the half-wave 4:1 impedance transforming balun also suffers quite badly from asymmetry, and the effect of the capacitive imbalance is exacerbated by the high impedance environment.
The best structure I measured was the so-called Pawsey Stub (it was actually first described by a guy called White working at EMI Labs in the 1930's FWIW) Made symmetrically from 0.141 semi-rigid, the losses were effectively unmeasurable, and the balance within 0.1dB.
There are a number of games which can be played with the White/Pawsey Stub. Firstly, it doesn't HAVE to be 90degrees long. In practice most dipoles - split or folded - have a a fringing capacitance in parallel with the feedpoint, which is often compensated by playing with the length of the dipole. That seems crazy to me. The White/Pawsey stub can be capacitively loaded, if the length of the stub is reduced to compensate, without significantly effecting its performance. In my case I took advantage of this to solder the centre of the straight dipoles to a small piece of low-loss PCB material (1.6mm Rogers 4350) thus giving me a mechanically solid dipole centre and shortening the length of the 1:1 balun. The losses were very small, and thus difficult to measure. As an aside, there's clearly an optimum point where the losses due to the loading capacitor equate with the improvement in insertion loss gained by shortening the coax lines....!
The bandwidth of capacitively loaded baluns isn't drastically reduced, by moderate loading, which also demonstrates that losses are unlikely to be a problem. The bandwidth of the balun was also significantly greater than that of the straight rod dipole I was using.
The second trick which can be played is to turn the 'active' side of the balun into a 90degree impedance transformer. This implies either that air is used as the dielectric, or that the balun structure is capacitively loaded. Transforming baluns using this technique would be a very nice way of feeding yagis with low impedance feeds, and suitable baluns could be made with multiple lengths of cable. As an example, It would be quite easy to make a balun/impedance transformer to feed a '28ohm' yagi using parallelled lengths of 50 and 75 ohm cable. For a transformation from 50 to 200ohms, it's not difficult to envisage a structure using stripline (NOT microstrip!!!) or slabline.
On 7-Oct-2004 DL4KG wrote:
If you want to see a well done quarter wave sleeve balun,
look here: http://www.qsl.net/dl4kg/image.html
It was made by PA4FP according to a DUBUS article written by DJ9BV. I used it only for a short time before a storm destroyed my open feeder system last year. Nowadays I am giving up on 432 MHz EME due to strong local noise. If anyone is interested I am willing to sell the balun. The double slug tuner is also available. Please email me if interested.
(Note: I'm not showing any E-Mail
address here in order to avoid them from being collected by SpamBots. You can
possibly find the E-Mail addresses of the above OM at QRZ.COM.)
This discussion is still not closed. If you have any opinions or additional
related information you would like to be published here, just send
me an E-Mail
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