Multiband end fed Zepp Antenna loading problelm

[K8JLY]

I have constructed and erected and End Fed Zepp antenna according to information obtained from the 1946 ARRL Handbook. The design described calls for an antenna length of 133' with feeder length of 45'.   Due to the location of the transmitter and other considerations my feeder length is 49'-6" from the antenna to the antenna tuner at the transmitter.  The feed line is 600 ohm consisting of #12 stranded copper separated by Birnach 6" porcelain spacers at 30" intervals.  The antenna is 28' above ground level at the lowest end and slightly higher at the feed end.  According to the text this antenna should work on 3500 kHz with parallel tuning and suggests series tuning will be best on the 7000 and 14000 kHz bands.  The vintage transmitter I am using has (2) 250TH's in the final and easily loads to 800 watts into a dummy load. Link coupling is used between the final amplifier and the antenna tuner which is mounted above the final in the rack.  My operating frequency at the moment has been 7280 kHz.  With series tuning as suggested I get no load (current) to the antenna no matter what taps are used.  I configured the tuner to parallel feed and was able to get the final to load to about 600 watts but only showed 1 ampere of antenna current.  The antenna is ungrounded. I have checked the tuner with a grid dip meter and confirmed that it would resonate at 7280 kHz.   I have reviewed the information in the handbook(s) and read that I should be able to match my antenna configuration.  Any help is greatly appreciated.
tnx... Cid

[KA0HCP]

Cid,
I checked my copy of the 1946 handbook Page 212.   The figures you quote for the ideal antenna do not match those in Table 1.

Further, you have modified (by necessity) the feedline length significantly.

Keep in mind that Zepp antennas (including feedline) are a tuned "System".

*I did a quick run on EZNEC with the figures from Table 1 of: 136ft antenna, 67ft feedline using  your 6" and 14AWG wire.   With the antenna at 72 feet (feed point 5ft above ground), the SWR bandwidth on 7Mhz is quite broad and should provide an SWR of less than 1.5:1 into a 600 Ohm load; an easy match.

Using your dimension of 49.5Ft feed and 133ft antenna with same feed and antenna at 54 ft, the SWR is extremely high and not a good match at 600 Ohm.  Very high losses.

I can't easily model your precise setup.  Having the antenna lower at 28ft would have some noticeable effect due to greater ground coupling.

Recommendation:  Follow the dimensions exactly as provided in the Handbook
-Add 3ft to your antenna length, 136ft total.  This can be done easily just by adding 3ft and letting it hand down vertically.  
-Increase your feedline length to 67ft.  Use any combination of rope or hard point supports as needed.  Of course, OWL can't be curled up like coax or allowed to be within about 4-6 widths of itself or metallic objects.

Good luck!  Just a few simple changes and you should have a good signal output and easier loading!  bill
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[KA0HCP]

1946 ARRL Handbook Page 212.

[K8JLY]

Thank you for the information....  The transmitter is located on the second floor of my building.  The feed line enters approximately 18' above grade and as mentioned the antenna is 28' above grade at the feed line junction.  I am familiar with the chart you provided.  It would be no problem to add 3' to the antenna.... The problem is with the feed line as I would have to add approximately 15' to get to the recommended length.... Are there any other options that I could explore with this type antenna with the 600 ohm ladder feed line? 

tnx... Cid

[KA0HCP]

A Zepp is a tuned system....Half wave antenna fed with a 1/4 wave feed.

As I mentioned you could use ropes to support the extra feed length.  Others have made up 'peg boards' from wood with wooden dowels/peg to provde a lengthy "S" pattern" in a short area to support extra feed lengths of OWL.

Change configuration it to a center fed dipole.  Dipoles are a bit more forgiving and don't necessarily require a tuned feed length.

The other alternative is to change to a the shorter 7Mhz  Zepp or dipole, with a 67 ft antenna and 33ft feed, per the chart.   Loading will be much easier with a resonant antenna for 40m rather than at a harmonic.    Performance would be much better.   

You would give up 80m, but then there wasn't a good match to begin with, since the design is a multi-band compromise.



bill

 

[KA0HCP]

Cecil, W5DXP is an engineer who has a number of good thoughts on antennas.
http://www.w5dxp.com/

Handling excess line with spiral on line:

[W4EWH]

A Zepp is a tuned system....Half wave antenna fed with a 1/4 wave feed.

I'm going to go out on a limb and admit that I have never understood how a Zepp could work. I'm not criticizing you, mind, just admitting to a lack of expertise.

I agree that a Zepp is a tuned system, but I always figured that the "tuning" included the capacitance of the operator's body, the metal frame of the aircraft, the incidental capacitance of the battery plates, and anything else that would make up for the lack of a radiator on 1/2 the feedline.

The best I've come up with, trying to visualize a Zepp's electrical properties, is to think of it as a "L"-shaped radiator over a "ground" composed of one of the feedline wires. I wish there was a better way to picture it, and if you have time, please tell me one.

Thank you.

Bill, W1AC

[KA0HCP]

1.  An end fed half wave antenna has a high impedance.

2. The original poster, cut his wire short making the impedance even worse.

3.  A quarter wave transmission line shows the same same impedance as the load (antenna) connect at the far end.

4.  Our friend has grossly shortened the feed line, causing even higher impedance at the transmitter end.  The consequence is his radio can't compensate for the extreme high Z.

A half wave antenna fed in the center (a dipole) has a low impedance, typically 50-72 Ohms.

With modern antennas we take advantage of this low impedance by using coaxial cable with a  characteristic Z of about 52 Ohms.   Coax can be any length and the impedance does not change. The trade off is coax is very lossy at any impedance significantly different from 52 Ohms.   

Open wire line has the advantage of not being so lossy with high impedance mismatches.  However, OWL will change or transform impedances, with changes in its' legth.  This can be a problem, such as this case, or it can be use to an advantage for matching certain antenna types.

[K8JLY]

Some additional information and questions concerning the Multiband End Fed Zepp Antenna.  I took measurements to certify the exact length of my feed line as it is presently installed.  The 6" spaced ladder line from the antenna to the feed-thru insulators is 40'.  The total length of the feeders from the feed-through insulators, through my antenna grounding switch and bus work to the terminals on the transmitter antenna tuner is 12'-2".  This makes the total feed line length 52'-2.   The 6" spacing is maintained through the grounding switch and all feed lines.  Following the recommendations provided by Bill (I need to add 3' to the overall antenna length and use a feed line length of 67') meaning I need to add 14'-10 more ladder line (not impossible).  I mentioned an article that I thought was in the 1946 ARRL Handbook, but it was actually in the 1934 Handbook.  Page 183 describes the Zeppelin antenna I was trying to duplicate.  Figure 1210 shows an antenna length of 133' and feed lines of 45'.  I found out I could not get to the transmitter antenna tuner with the recommended 45' feed line.  Forty-five (45') does not match 1/4 or 1/2 wavelengths on either 80 or 40 meters.  So how did this arrangement work?  The schematic shows a pair of capacitors designated C1 for series tuning and another capacitor C2 for parallel tuning.  I would guess you use one or the other.  The adjacent table (Table-I) provides some suggested feeder lengths for the Zeppelin and indicates a feeder length (each wire) of 60'.  Does this make any sense?  I appreciate any and all comments on this topic. 
tnx.... Cid K8JLY

[K3ZS]

[quote author=kb4qaa link=topic=36922.msg283219#msg283219 date=141003405

3.  A quarter wave transmission line shows the same same impedance as the load (antenna) connect at the far end.

[/quote]
From your other comments, you probably mean that a quarter wave section (with velocity correction) of line transforms a high impedance to low, a half wave reflects the same impedance.

[KA0HCP]

*Ooops!*  Thanks Robert.

Cid:  Yes, having additional capacitance to compensate is appropriate.