Quick Question About Coax..

[n3lrx]

I'm looking at some coax. Specifically RG-8X, Should I get 25 or 50 foot? I't will be for 75m. It's going to be curled up on the balcony, I'm just looking at a reasonable length to bring the SWR down. I've also got to borrow an MFJ-259 from a friend to get the SWR at the best possible and let the tuna do the rest. I can't tune up for scrap with the 9ft RG-213 I have. Obviously it's too short. Plus I'll be adding a choke to the antenna.

[Steve - K4HX]

The SWR is a product of the mismatch between the antenna and the coax. That cannot be changed by adding coax. The SWR may appear to be lower, but that is because of the increased loss from the additional coax.

However, adding coax may change the reactive component of the impedance the tuner sees and it may be a value the tuner can more easily handle.

You may want to check out the possibilities with a transmission line calculator like TL. There is also one on line.

http://fermi.la.asu.edu/w9cf/tran/

[W4EWH]

I'm looking at some coax. Specifically RG-8X, Should I get 25 or 50 foot? I't will be for 75m. It's going to be curled up on the balcony, I'm just looking at a reasonable length to bring the SWR down. I've also got to borrow an MFJ-259 from a friend to get the SWR at the best possible and let the tuna do the rest. I can't tune up for scrap with the 9ft RG-213 I have. Obviously it's too short. Plus I'll be adding a choke to the antenna.

First, I echo what K4HX said.

Second, consider that

• If you're going to match an antenna, the best place to do it is at the antenna. That's why broadcast stations have huts at the bottom of their towers: it's where the matching networks live.
• Coax is "OK" for use on 80 meters, if it's matched to the antenna. If not, you'll loose a lot of power heating up the the ants.
• If you have to use an antenna length that's sub-optimal, it's best to use open-wire line to feed it, because OWL can have a high SWR without the high loses you'd get with Coax.
• Coax is too convenient. There was a lot of it available as war surplus after WW2, and hams found out it was quicker to set up and use than OWL, and the rest is history. There's nothing magical about it: it's a compromise between setup speed (critical in a war) and "good enough" performance.

Sorry to bear bad news, but the match needs to be at the antenna if you want to use coaxial cable for the feedline.

73,

Bill, W1AC

[n3lrx]

A fine mess. The app won't run on Open Java, I'll have to install the evil Oracle Java..

[n3lrx]

Here's what I've got.. (it won't let me attach images. Maybe they're too big..)

http://i.imgur.com/vbgwg1r.jpg
http://i.imgur.com/lYQR20x.jpg

It consists of the following:

http://www.mfjenterprises.com/Product.php?productid=MFJ-347
http://www.mfjenterprises.com/Product.php?productid=MFJ-2375T (x2)

I wasn't planning on using these for transmit. They are limited to only 250w PEP. I can't run the larger version (also limited to 250w PEP) of this because one of the legs will extend into the neighbors space. I've been feeding it with 9ft of RG213 and just can't get the SWR down enough to even tune it with the tuner. The SWR watchdog kicks the transmitter off even in tune (70w).

I don't have an antenna analyzer so I can't dial in the antennas first. I know someone who's got one that might let me borrow it and/or him and his MFJ-259. If I can dial it in to something the tuner can knock down I might actually be able to get on the air.

I'm going to order a choke and install that so I don't get RF coming back down the line. I'm hoping that will make a difference. I was going to build one using RG213 and 4" PVC but I priced what it would take to buy the parts and my time, but I found 2 on ebay cheaper than I can build myself.

[K1JJ]

As Bill said, the problem with trimming a coax-fed antenna is that the coax length will add reactance to the readings and make it difficult to know when the dipole flat top is the correct length to present 50 - 70 ohms R at J0.  We need to make our measurements at the dipole's feedpoint, which can be difficult or near impossible without a bucket truck... :-)

Additional help to find the correct flat top length:  

A technique I have employed is to use a 1/2 wavelength coax feedline, even if temporary, to tune the proper dipole or Yagi driven element length. Figure in the coax velocity factor.  

Example for a 3.9 Mhz dipole test coax:  492/ 3.9 = 127'.  127' X .66 coax velocity factor = 84' of coax to equal true 1/2 wave on 3.9 Mhz.      So if you connect an 84' length (or X2...168', etc) of RG-213 (or RG-8) to a ~127' dipole, the MFJ-259 impedance measurements will be accurate.    After this dipole trimming is completed, you could then attach ANY RG-213 coaxial length and know that the match is good.

In fact, once the antenna is trimmed this way to true resonance, the coax length should have little effect on impedance readings, depending how close the dipole is to 50 ohms, J0 when resonant on 3.9 Mhz.  

Another way to get a true 1/2 wave coax:  Connect the MFJ-259 to the end of the ~ 84' coax. Short the opposite end of the coax.  Since a 1/2 wave will reflect back the exact impedance at the opposite end, trim the coax length until the MFJ-259 shows near zero ohms at 3.9 Mhz or the desired frequency for the dipole test..  We now have a true 1/2 wave coax adjusted for velocity factor.


T

[KA0HCP]

Yes, it's not a surprise you are having problems matching that hamstick dipole.   With it mounted within two inches of the overhead, it is being massively detuned by the structure.  

In addition, they have a very narrow bandwidth to start with.  If they are 75m sticks you are probably lucky to get 25khz - 50khz bandwidth in an ideal open area mounting.  

You man not even be able to get them resonant within the band.  Don't forget that if you shorten the wire, it cannot extend down into the windings with out also adversely affecting the resonance.  (This is one of those little know points most hams miss).

A good choke is absolutely necessary.  A commercial ferrite choke will perform far better than an air wound coax choke (and be useable on multiple bands, vice one).

9 feet of coax vs. 25 or 50 ft is not going to make a significant difference.

I learned these same lessons when I tried to scale down a 20m yagi to hang from the concrete ceiling in my dorm room years ago, and while also trying to use magnet wire glued to the brick facing.    Capacitive detuning from structures is a bear!

I wasn't planning on using these for transmit. They are limited to only 250w PEP. I can't run the larger version (also limited to 250w PEP) of this because one of the legs will extend into the neighbors space.

Are you seriously planning on running 250w transmit in an apartment/condo?
That is a guaranteed way to annoy every neighbor and get shut down pronto.     Stealth, guile and low power are the strategies for apartment operating!

Good luck, bill

[AB2EZ]

Antennas can be pretty mysterious for people who haven't spent time studying them and/or modeling their behavior

Since your dipole antenna appears to be much shorter than 1/4 wavelength on either side of the feedpoint (i.e. much shorter than 1/2 wavelength total  length), it will have the following characteristics:

A. Even with a tuner and/or other matching network, it will be a very inefficient radiator. Only a small fraction of the transmitter's output power will be coupled to radiating electromagnetic modes. I.e. coupled to waves that propagate to distant places.

B. Most of the power that enters at the antenna's feed point, and then leaves the antenna, will couple to near field electromagnetic modes.. and then will be absorbed, and converted to heat, by currents produced in nearby conductors (wires in the walls, steel structural elements, the ground). You can anticipate a lot of RFI, and even arcing between conducting objects that are in close proximity.

This is a consequence of the fact that an electrically short / very short antenna has a high / very high ratio of near field strength (volts per meter) to far field strength... compared to a full size antenna.

For example, if the total length of the dipole is 1/8 of a wavelength (1/4 as long as a full length dipole), then: to obtain the same radiated power, the current in each of the two dipole sides will have to be 4x as large as it would be in a full sized dipole. This is just a consequence of electromagnetic field theory and the mathematics of coupled mode theory. As a result, the near field strength (volts per meter) will be 4x as large as it would be with a full size dipole radiating the same power. This is also a consequence of electromagnetic field theory, and of coupled mode theory.

This, in turn means that the resistive losses associated with currents flowing in nearby conductors (including the ground) will be 16x as large as they would be with a full size dipole, radiating the same power.

If there are no nearby conductors, or if all nearby conductors... through which current can flow... have very low resistance, then the near field losses will be very low for a full size dipole, and will still be low when the near field strength is 4x as large.

However, if one considers a typical case where ground losses are not negligible, and other losses produced by near fields are not negligible... then the effect of increasing the near field strength by 4x is very significant.

Consider a case where one is using a full size dipole over typical ground, and where about 25% of the RF power, that enters the antenna at its feed point, is coupled to the near field modes... and then converted to heat. About 75% of the RF power, that enters the antenna at its feed point, is coupled into the traveling (propagating) waves of the far field.

I.e. for every 3 watts of radiated power, there is 1 watt of power converted to heat by ground losses.  

If the antenna is 1/4 of full length, then (after pushing all of the power into the feedpoint of the antenna, using a matching network and/or a tuner)... for every 3 watts of radiated power, there will be (4x4) x 1 watt =16 watts of power converted to heat by ground losses. I.e. only 3/(16 + 3) = 16% of the total power entering the feedpoint of the antenna will be radiated into the far field.

If the antenna is only 1/8 as long as a full size dipole,  then only 3/(64 + 3) = 4.5% of the total power entering the feedpoint of the antenna will be radiated into the far field.

If the antenna is only 1/16 as long as a full size dipole, then only 3/(256 + 3) = 1% of the total power entering the feedpoint of the antenna will be radiated into the far field.

C. The power handling capability of the antenna itself will likely be much lower than specified... because the currents being pushed into the antenna elements (by the tuner and/or matching network) will be much larger with this electrically short dipole than they would be with a full size dipole.

Bottom line:

If you want to use an indoor antenna for transmitting (as I do), you will have much better results if:

You are able to manage to put up a dipole or an inverted V whose total length is at least 0.25 wavelengths (at least half as long as a full size dipole.

[My indoor antenna is a wire inverted V that is 25 feet long (about 7.5 meters long) on each side of the feed point. The total length is approximately 0.5 wavelengths on 30 meters (i.e. a full size dipole), but only 3/8 wavelengths on 40m. I use it on 40m, 30m, 20m, and 10m. It works well (I get good signal reports) on those bands. I can load into it on 75m, but nobody can hear my signal.]

Make sure that your house or apartment is not enclosed with a steel skeleton

Stu

[K1JJ]

Part 2...

Now what if we have an existing dipole (or any antenna) fed with a random length of coax... how do we determine its true resonant frequency without playing with a 1/2 wave coax as described above? IE, we want to hook the MFJ-259 up to ANY coax-fed dipole as is...

First:  Remember that when using a good 50 ohm dummy load, one without significant reactance, we can use ANY length coax and see the same 50 ohms R and J0 readings.  Now if we measure a reactive dummy load, the inductive and capacitive reactance readings will vary depending upon the coax length.

So, if we have a dipole that happens to be truly resonant on 3.9 Mhz, by adding some coax 20' or 40' jumpers to the coax, (1/8 or 1/4 wave jumpers) we should see little L/C reactive reading changes on the MFJ-259 ant analyzer.  But if we move the MFJ-259  frequency down to say, 3800, the reactance readings will vary greatly when we change coax jumper lengths.  

So, a way to test an existing dipole with an unknown coax length is to simply add and subtract test coax jumpers and find the frequency where the reactance readings change the least with coax length changes.

I have used this technique to double check  Yagi antenna systems once in the air and finished. When an antenna is finished, we may have who knows what coax lengths and don't really care...

T

[n3lrx]

Nah, I won't be running 250w more like half of that if I can get the transmitter down that low without sounding yellowy. But of course what would be said if YellRX isn't yellowy.. lol Besides I don't want any talking toasters.

I'm not too concerned with the bandwidth if I can get 3880-3885 I'll be happy. I can't ask for too much in my situation. I just have to make due with what I've got until another opportunity to buy a house with the real estate arises.

I'll also check into a ferrite choke.

I wish I could set up an antenna like an end fed, a sloper, or inverted L, but I can't the office says no antennas. Even one that can be practically invisible and only requiring a small hook in the wall for one end and a tree for the other. I see hanging flower pots but I'm not allowed to screw a hook into the wall for an antenna.

Oh well..

I need a proposal written. Anyone good at that? I need one that can make amateur radio shine so the office will let me put up an antenna.

[ka4koe]

I think even a tuned loop antenna inside your house would work better than a hamstick dipole. It would run around the perimeter of the largest living space inside....course, the voltages on it due to reactance would dictate care in its installation.

OR an attic dipole made as large as possible each side (but equal).

In any event, please refer to the QST article "Antennas and Grounds for Apartments", Dec 1980, Peter Dell, AE8Q, or get a book from the League at

http://www.arrl.org/shop/ARRL-s-Small-Antennas-for-Small-Spaces/

Good luck. You must have MORE radiator!

Philip KA4KOE

[Pete, WA2CWA]

Or just Google: Antennas and Grounds for Apartments

[WBear2GCR]

I'd push the emergency communications idea. Power outages, natural disasters, terrorists, etc. Mention New Orleans - the only communications ended up being hams.

Would be nice to see the situation, perhaps some images of the apartment outside, and the area would show us what is going on...

If you have $$ to spend, and the situation is appropriate, perhaps they would like a nice *flagpole* not far from your apartment?? You would "donate" the flagpole and the installation.

That would permit shunt feeding, "J" and "L" type ants, as well as just as a vertical. A parallel wire would do as a vertical too... you could bury the coax in some PVC waterpipe (cheap) or PEX tubing, nobody would see a thing.

                            _-_-

[n3lrx]

 I'll see about getting some pics. As far as the emcom goes, other than my 2m/440 HT and my 2m mobile I'm off the air in a local power failure. My HF consists of a K7DYY 160/80m transmitter, my audio chain (A minimum of one unit), and a DX300 receiver. The receiver works on battery power but the rest of my HF gear I'd need a generator or significant battery backup to run. (I have neither)

 I'm also in the back of the building which is good because of the trees. But donating a flagpole won't do much good in my case. I guess there really is no shining example as to how I can use ham radio in the midst of a natural disaster, but someone who's good at writing up proposals might be able to polish something up. (VJB where are you my friend? )

 I've already got a diagram drawn up as to how the antenna would be mounted using an end fed wire as an example. It shows only a small hook in the building and the rest of the support being a tree. People have flower pot hangers screwed the the building. I'd cause no damage to the building other than the hole of a single screw. I've already for the hook in place. I'm also on the third floor of a 3 story building so I have at least a little bit of height. Not anything special, but it's better than being on the first floor or sandwiched in the middle.

 I'll check into those pictures. My media card reader has been having issues lately so it's time to get a new one. it's hit or miss getting pics off my camera, and I'm on Linux, which my camera won't work with it directly. Something in the new kernels has hosed that. I've got a bug filed on the issue.

[WD5JKO]

Randy,

    I think your pushing a wet rope trying to convince the housing complex folks about why having an antenna up is in their best interests. Trying to put the "public service" aspect of amateur radio above the real lightning hazard, and the tenant interference likelihood will be tough unless your dealing with someone who really likes the idea of ham radio.

    In the past I once lived in an apartment on the second floor, and there was a tree about 60' outside my window. I put up an antenna late at night using something like 28 awg magnet wire. The antenna was invisible. Maybe it was wise or unwise but I decided to forgo getting permission, and instead took the approach to just put it up. If it became an issue, I'd beg for forgiveness while taking it down, "oh sorry, I didn't know"...But I was a kid in high school. In your case it's too late since they are watching you now that you asked, and they said NO.

   As already mentioned a small loop has possibilities. I'd ditch the K7DYY and trade it for a multi-band rig that can get you 100 watts on the upper bands. A simple 16' X 8' loop might fit that balcony, and that would be a full wave on 15 meters. You could coax feed it on 15m. With OWL, it would work 20-17-12 meters. Open the corner opposite the coax and it will play on 10m. I have one of those loops mounted vertical with the bottom about 8' off the ground. It works pretty good on the upper bands. With a EFJ 275W tuner it works on 40m as well. But with coax only, and no tuner, 15m works well.

Jim
Wd5JKO

[n3lrx]

 Heck no, I'm not going to get rid of my K7DYY rig. I'll save up and buy another HF rig before that, or just wait until I move out. I've waited and dreamt of owning a DYY rig  since Bruce came out with his first transmitter, the 'Junior', then the 'Senior' came out, and now I finally own a 'Super Senior'.

 I bought it last year. Since then I've been talking to a dummy load to tune up my audio. It's the first full power rig I've owned and unfortunately, for my present situation 75 and 160 are the only 2 bands I really have interest in.

 I've asked Bruce if he had any plans to make a 40/20m version but he didn't answer. So I take it as a no. If he did, I'd buy one.

 My balcony is even too small for any of the reasonable loops I've seen. It's a small balcony with an aluminum railing using up at least half the vertical space. I had previously thought it was wood, but found out when I moved in, it's definitely metal.

 This is also a temporary problem. I moved here to get out of the single room apartment I was in for nearly 7 years, so it's an improvement in my life, but within a few years I'll probably be either buying a house here in Michigan, or moving south to Texas. I have 2.5 acres promised to me to build on if I choose to move there, that's plenty of room for some wire in the air.

 The good news is I actually was able to load the micro dipole on 3880 and get a 1.8:1 match. I haven't turned the carrier down yet so I've only used it in the 'tune' mode, 70 Watts with no modulation. I don't dare dump 375 watts into it I might smoke something. I'll try to load it later after turning the carrier down and the neighbors go to bed.

 The bad news is in the process of tuning up I blew up my wall wart for the meter backlight. It's the second one I've killed.. These things don't like RF very much. The next one will definitely have a choke on it. I might just replace it with a heavier one. I hate to use my Astron 35A for a single LED so bigger wall wart it is.

 I still can't hear much right now the noise is pretty high, but in the past I was able to hear the Woodlawn Powerhouse at s9 off this micro antenna. Heck, Don even comes in s9 with a 12 ft. hank of wire coiled around the curtain rod.

 I do hear a few on now on 3885 as a matter of fact off the curtain rod. A few at about s5 peak, which is right above the noise. With all the fading I can't hear any calls or hear enough of the voices before they fall below the noise to recognize who they are. One is definitely Don. he's has risen above the noise!

 Anyway, I have enough educated opinions here to make an educated guess that I'm screwed.. lol

 Thanks all for the advice!

[K1JJ]

The bad news is in the process of tuning up I blew up my wall wart for the meter backlight. It's the second one I've killed.. These things don't like RF very much.  

I lived in urban Denver in 1974, in a one room studio apartment. The four story building housed 8 units per floor, like a giant beehive.  I sneaked an inverted V for 75M up on the roof at 60' and ran  quad 4-400As in a badass homebrew linear. The antenna could not be seen from the ground due to the flat roof and surrounding buildings.

Needless to say I was destroying the TVs and stereos in the complex. One night I was talking to the guys in W1-land on 75M and someone in the apartment unit below me slammed a pipe into the ceiling (my floor) to show his displeasure. At the end of the month I got an eviction notice for: 1) having a dog, Yaz I    2) interference (RFI)   3) making "drilling sounds" at 3AM in the morning...

We all pay our RF dues somewhere along the line.  Good luck with the project, OM.

T

[AB2EZ]

If you listen to you own signal with a local receiver...even if you are not modulating your signal... I suspect that you will hear a large amount of hum modulating your carrier.

The reason for this is as follows:

Your transmitter is, in effect, trying to find a lower impedance path, for RF to flow into, than your very short dipole.

The path is from your transmitter to the house wiring... via the wall wart, and also via the AC power cord.

Isolation transformers and ferrite chokes will typically not produce a high enough impedance to stop the RF from your transmitter from entering the house wiring.

Various non-linear effects will cause the 60Hz AC to mix with the RF flowing through the house wiring (modulated or not)... resulting in a 60Hz modulated RF current flowing in your house wiring.

The wires in your house are a better radiator (antenna) than your very short dipole.

Therefore, the dominant signal you will radiate locally (near field) and into the ether (far field) will have a strong 60Hz hum on it.

This same phenomenon (RF being pushed from the transmitter into the house wiring) is probably what is burning out your wall wart.

I ran into a similar situation when I was experimenting with a little SSTRAN (brand) AMT3000 (model) 100mW AM broadcast band transmitter... to use it a send music to AM receivers around my house in Bernardsville NJ.

FCC rules for these unlicensed AM broadcast band transmitters limit the length of the antenna and the ground plane wire, supplied with the transmitter, to something like 1.5 meters (each). In effect, this is just a very short dipole.

[Note: the purpose of this FCC rule, regarding the antenna length, and the ground plane wire length, is to make the unlicensed AM broadcast band transmitter radiate its power almost entirely into the near field... with extremely low coupling to far field modes.]

I experienced lot's of 60Hz hum on the signal when I listened to it with my AM receivers.

At first, I thought the 18VAC wall wart, combined with the internal rectifier of the AMT 3000, was producing the hum... but I quickly ruled that out as the source of the hum. Then, I tried various approaches to blocking the RF from getting into the house wiring via the wall wart (the wall wart powers the entire AMT3000, which can operate from an AC or a DC external power supply). I tried different, filtered DC power supplies; I tried RF chokes, I tried ferrite cores. None of those approaches reduced the hum.

Finally, I hooked two 9V batteries in series to substitute for the wall wart power supply. The hum completely disappeared.

Stu

[W4EWH]

Here's what I've got..

OM,

I didn't realize that your space was so limited.

I suggest you investigate renting storage space and roof rights at a nearby building, put up a full size antenna, and run your rig over the Internet via remote control.

You might also be able to trade use of the rig with another ham in return for remote access.

HTH.

73,

Bill, W1AC

[K4RT]

How much room do you have to work with? What is the square footage under that eave, and how high is the eave above your deck?

If you can't get the hamsticks to work on 75, you might have to choose a higher band(s) to use while at that location, and hang a folded folded dipole under that eave.  I'm not saying 75M is out, but if it turns out that way, try higher bands.  Next hamfest you go to pick up a used ARRL Antenna Book, which contains lots of information about antenna construction.

You might consider trying modes other than AM for now that will be more neighbor friendly.  You can do a lot with 10 Watts on PSK31 or CW.  You can build your own PSK31 soundcard interface and the software is free. Try AM and maybe no one will complain. If the only rig you have now is the K7DYY and RS receiver, hold onto those for AM, and keep your eyes open for a good used SSB/CW transceiver.

We lived in a 2nd floor apartment with a deck for 5 years and after trying several things that did not work, I found that a folded dipole for higher bands hung from a rain gutter over our deck (fed with RG8X) worked. There was coupling, which threw things off, but experimenting and carefully pruning the dipole legs brought the antenna into resonance, and it wasn't so lossy that I could not work into Europe, South America, etc. on CW and SSB running low power.  The first year we moved to Virginia, we lived in a ground floor apartment. At night I ran a 50 ft. run of RG8X to the Hustler mobile antenna on my car and that worked well for 40-10M.

Good luck - you'll figure out something that works.

[Steve - K4HX]

Good point RT. Forty meters might be a better target for several reasons.

1. Shorter wavelength - less size required for antenna
2. As in #1, antenna does not need to be a high
3. Even with a full-sized antenna, lower power fares better on 40 meters than on 75 meters - more favorable propagation and generally less QRM
4. May be more local activity on 40 meters.

The only downside is that there is little night time activity on 40 meters, and that could be a problem, depending on your schedule.